I have also experienced that the phage 'drop' the inserts if you keep re-growing the same initial stock. It starts with minor 'breaks' in the insert and then no insert at all. You will have to keep growing from glycerol stocks to ensure your library is intact. There was a paper on this that I recall reading but I can't seem to find it now. I think it came from the Smith laboratory itself. There were guidelines on how many generations you can have without losing the insert.