SH-SY5Y cells do not grow - (May/14/2012 )

Hey all, I need help

I transfected SH-SY5Y cells with a protein in a pCDNA vector and selected them using geneticin 250µg/ml.
1) the untransfected control plate died 1-2 days later - on the transfected plates only about 80 % died and the rest looked well.
2) the transfected cell do not grow. they are 2 weeks under selection now and seem to be fewer from day to day.
3) the untransfected cells that are not under selection still grow well, so its not a problem of the medium (AMEM, F12, FBS, Pen/Strep, L-Glu)

Where could the problem be???

What density are the cells at? Selection only works when the cells are actively growing, this means that you typically need them at less than 70% confluency.