There's a useful distinction between a late invoice and a lost one.
A late invoice is a problem you know you have. It's on a list somewhere, it's overdue, and somebody can chase it.
A lost invoice isn't on any list. You find out about it when the supplier rings, or when they put your account on hold, or when it turns up in a year-end query about an expense nobody recognises. By then the delay is the smaller half of the problem — the larger half is that your accounts were wrong in the meantime, and you didn't know.
The two have different causes and different fixes, and it's worth being clear about which one you actually have.
Lost means there's no record it exists
An invoice can only get lost if there's a point in the process where it exists as a document but not as a record.
That's the whole mechanism. Between "a supplier sent this" and "this is a row in the ledger" there's usually a gap, and anything in that gap is held only in someone's inbox or someone's memory. Nothing is counting it. Nothing will notice if it never comes out.
Once an invoice is a record — even an unapproved, uncoded, obviously incomplete one — it can be late, queried, disputed or wrong. It can't really be lost, because something is keeping score.
So most of the answer is about shortening that gap.
Where they actually disappear
Sent to a personal inbox. A supplier emails whoever they last dealt with. That person is on holiday, or has left, or simply doesn't handle invoices. The email is not forwarded, because from their point of view nothing happened.
Buried in a thread. The invoice is the third attachment on a reply to a quote, under a subject line about something else. Nobody searching for invoices will find it.
Sent to an approver and never returned. The most common one. Finance forwards it for sign-off. The approver reads it on a phone, means to deal with it, and it moves down the inbox. Finance's copy is in Sent, which nobody reviews. Nothing anywhere says "waiting".
Left in a shared inbox with no ownership. A shared address seems like the fix, and it helps, but a shared inbox with three people and no convention about who processes what produces its own losses — everyone assumes someone else picked it up.
Approved verbally. Someone says yes in a corridor or on a call. The approval is real and the record isn't. It surfaces months later as an expense nobody can substantiate.
Attached to a person, not a role. The one person who knows how these get handled is off for two weeks. Nothing is lost while they're away, but nothing moves either, and some of it falls out entirely.
Every one of these is the same failure: the invoice was in transit, and transit is where there's no record.
Why email approval is where most of them go
Email is the default because it's free and everyone has it. It's also close to the worst possible tool for this, and specifically for one reason: it has no state.
An email is sent or not sent. There's no such thing as an email that is "pending", "with Dave", or "outstanding for nine days". The information about where things stand exists only as a set of messages spread across several mailboxes, and no one person can see all of them.
Which produces the characteristic symptom. Ask a small finance team what's currently waiting for approval and you'll often get an honest "I'd have to go through my sent items". That answer is the problem stated precisely — the queue isn't visible, so items can leave it without anyone noticing.
The related trap is that silence looks identical to progress. An approver who hasn't responded and an approver who is about to respond produce exactly the same amount of information, which is none.
The cost, beyond paying late
Your accounts are wrong. An unrecorded invoice means understated costs and overstated profit for the period. If it's material and it crosses a year end, it's an adjustment.
Supplier relationships. Suppliers rarely mind being paid on terms. They mind not knowing. Repeated chasing for invoices you've apparently never received tends to shorten terms or tighten credit.
Duplicate payment risk. A chased invoice gets re-sent, and a re-sent invoice gets entered fresh. If the original then turns up, you have two records of one liability and a decent chance of paying both.
Lost early-payment discounts, which are usually the least of it, but they're the easiest to quantify if you need a number.
What actually stops it
One address that everything goes to. The highest-value change, and it costs nothing. Suppliers send to a single dedicated address rather than to individuals. Tell them once, then put it on your remittances and purchase orders so it keeps being reinforced. Personal inboxes are where invoices go to disappear.
Record it before you approve it. This is the important inversion, and it's the one most small teams have backwards. If the invoice becomes a record on arrival — unapproved, uncoded, incomplete — then approval is a status on a thing that exists rather than a prerequisite for it existing. Now nothing can be lost, only pending.
A queue somebody can look at. Not necessarily software. A shared list showing what's outstanding and with whom removes most of this, because it converts silence into a visible row that's been sitting there for nine days.
Ownership of the shared inbox. Whose job is it to empty it, and by when. A shared inbox without that convention is just a slower personal one.
A rule that verbal approvals get written down by whoever received them, immediately.
Reconcile supplier statements. This is the safety net rather than a fix. A monthly statement from a supplier lists everything they think they've sent you. Anything on it that isn't in your ledger is either lost or never arrived, and it's the only routine check that catches invoices you don't know exist.
The test
Ask one question: what's waiting for approval right now, and how long has each one been there?
If the answer comes from a list, invoices in your process can be late but not lost.
If the answer requires going through anybody's email, then you don't currently know — and "I don't know" is the condition under which invoices go missing. Not carelessness. Just an absence of anything keeping count.
Cribble records invoices when they arrive rather than when someone gets to them, so an unapproved invoice is a visible pending item rather than an email in transit. Approvals run through Admin, Finance and Approver tiers, and what's outstanding is a list.