How do you co-transfect DNA and siRNA into HEK cells?

I tried to co-transfect plasmid DNA and siRNA Silencer Select from Lifetechnologies into HEK 293. I used the protocol from the company and Lipofectamin 2000. So far I didn’t even get knockdown of the positive control (GAPDH) although I tried lots of different concentrations. DNA works, though. Does anyone have similar problems?