ich

I cant tell you for sure, but can give some theories. Ich can live in your tank with no visable signs. Stress (such as that caused by birthing) can cause an outbreak of visable ich. What are you doing to trat the ich when it occurs? It is possible to treat for ich, but not irradicate it from your tank, if the treatment is not carried on long enough. Here is an artcle explaining the life cycle of ich, and how best to rid it from your tank forever http://www.aquariacentral.com/forums/showthread.php?t=39759