Used stamps exchanges.

Hi,
I posted BE-415 to a stamp collector in France (together with another letter to Croatia). After 3 weeks I contacted the stamps collector to check if the letter had already arrived, but he replied that this was the case (letter to Croatia was received after 4 days).
It's been more than a month now since I posted the letter and still the letter hasn't arrived at destination. I can only assume that the letter has been lost by (probably Belgian) post.

What are the rules for this kind of situation?
Is the BE-415 to be registered as "missing"?
Do I re-send stamps to the other person reusing the old number?

Many thanks,

Thomas

The letters are marked as expired after 90 days. If a letter is lost, you can do a resend using the same ID. You can check with your partner first (as your did) to check if the letter arrived and if the member is still active on the website to register your resent letter.

*he replied that this was NOT the case* (obviously, small typo. Sorry for the misunderstanding)