293T cells not responding to FGF as expected!

Hey,

Having an issue with 293T cells and FGF stiumaltion assays. I grow 293T cells, transfect them with expression vectors containing proteins that i know increase response to FGF stimulation compared to untransfected cells, and i know that 293T cells endogenously express fgf receptor. i know the expression vectors express protein, via western blot, but the response to FGF stimulation after incubation in minimal media (krebs HEPES buffer) does not follow the expected outcome. when comparing even untransfected, non-fgf treated cells with fgf treated cells, no increase in pERK is seen (considered the measurable output for fgf stimulation), so the issue seems to be with the 293T cells or the Fgf, what do you guys think?? cells have been shown to be mycoplasma free.