As was said above, what were your OGs and what attentuation figures are you getting?
S-04 and US-05 are very reliable yeasts, you should be getting mid-70s attenuation figures, so if OG around 1045-1050 you should be getting down to 1010-1012. As your FG is around 1020 either your OGs are very high, or your yeast isn't working as expected, maybe due to lack of oxygen or something in the mash process. I think this is your problem.
S-04 and US-05 are very reliable yeasts, you should be getting mid-70s attenuation figures, so if OG around 1045-1050 you should be getting down to 1010-1012. As your FG is around 1020 either your OGs are very high, or your yeast isn't working as expected, maybe due to lack of oxygen or something in the mash process. I think this is your problem.