Why don't all trades complete?

I post a trade offering 10 eggs for 70 coins.

I get four replies.

I accept all four.

Three complete and one doesn't. I don't lose the eggs but I don't get the coins.

It doesn't seem to matter if I accept all four emails at once, or do one at a time.

Any idea why this happens and what I can do about it?

Thanks,
Richard