problem with embryo carcinoma cell line , p19 , - (Aug/30/2011 )

hi

i am phd student

i am working on p19 cell lines
in the beginning of my first year the p19 cell line was supper confluent nearly 80-90% within 24hr and they were in a good condition
however, nowadays i have problem growing them. they start growing very slowly and they detached from the flask easily.
i have not change the medium alphaMEM with boven calf serum 7.5% and FBS 2.5%

i do not know why they start behave this way

any recommendation or opinion about what may go wrong with p19 cell line

thank you in advance
ohoud

How long have you had them growing (when did you last thaw a tube)? Did you freeze them in DMSO? Have you tested them for mycoplasma?