only one plasmid expressed highly after co-tranfection into cells - (Apr/05/2012 )

I transfected one plasmid expressing protein of interest tagged with V5 and a plasmid expressing GFP together into MCF10A cells. I did immunostaining with V5 antibody but found most transfected cells only expressed one protein at high level. For example, cells with high GFP signal had only very low level or undetectable V5 signal, and cells with strong V5 staining only showed weak GFP. Only a very small portion (<10% of transfected cells) had both strong V5 and GFP signal. I don't know why. Could anyone kindly give me some suggestions? I used lipofectamine 2000 to do transfection.

Thanks very much

Can you tell us how you set up this experiment, so that we can trouble shoot?