What a returned envelope actually looks like
People picture a rejection as a letter with reasons. In practice a file that goes nowhere comes back looking almost exactly like the one that left: the same sheet, the same folds, the same handwriting on the front. Nothing shouts. The whole message is in what is absent from the paper, which is why a quick glance is not enough.
Why an Eritrea-bound file returns unused
Eritrea has not joined the Hague Apostille Convention, so its offices do not treat the British certificate as the closing act, and document legalisation for Eritrea runs through consular channels instead. The mismatch is structural. It has nothing to do with the condition of your sheet, the office that issued it, or how carefully the envelope was packed.
There is no finished object called an apostille issued in Birmingham for use in Eritrea, so a file built around that idea has no ending to reach. It can travel, collect a real British mark, and still arrive at a desk that has no way to accept it.
Three shapes of a file that failed
The first shape is a sheet with nothing added. That normally means there was no British public signature or seal to certify, which happens with plain photocopies and with papers signed by private individuals. The second is a sheet with a genuine certification on it that has simply stopped short of the consular desk, and is therefore incomplete rather than wrong.
The third is the awkward one: stages done out of order, so a later desk is looking at a paper that has not passed the stage it expects. Each shape has a different repair, and treating all three as the same problem is how people spend twice. Read the marks before deciding anything.
What the paper can tell you on its own
Line the returned sheet up against your own note of what you sent and check every mark for its issuing office. A new certificate means the British stage ran. No new certificate means it did not, and that points at the sheet rather than the route. Seeking an apostille from United Kingdom to Eritrea was never the workable plan, so the fix is usually the plan.
Then rebuild from the source rather than from the returned paper. A fresh certified copy from Birmingham Register Office at Holliday Street costs far less than a wasted consular attempt, and starting clean removes the risk of carrying an unnoticed fault forward through every later stage.