Sounds like whatever you're cloning may be inhibiting growth. Try
decreasing the amount of IPTG (I use 4ul of 100mM spread on top of
plate). Another thing you could try is adding a small amount of glucose
(0.2% I believe) to your plates. Yet another thing to try, in case it
is the cells gone off, is restarting your stock culture by streaking
out the XL1's on a tetracycline plate. This will ensure that all the
cells are carrying the F' episome.
sjeffrey at mskcc.org
What do I know, I'm only a tech!