I'm answering some parts of my own question :
as a matter of fact, the entry missing in my replica has an entryCSN lesser than the contestCSN. I guess that's the reason why it is not synchronised.
Does that mean that, if for some reason, the replica gets out of sync in a similar manner, missing entries will never get synch'ed again unless - by "chance" - touched (modified) again (in which case they'll get a new entryCSN) ?
Of course, I still don't undestand why this entry disapeared in the first place...
Thanks
-- Thomas HUMMEL