Clippers not shaving my dog clean..?

I have a really nice pair of clippers they are Osters. I have a Collie who has a very thick undercoat.I started shaving her and my clippers keep just barely clipping, I just picked them up from getting them sharpened so they are not dull. When I have clipped my horses during the winter we always used show sheen and stuff like that to make the hair easier to clip.Is there anything I can put on my dog or some step I am missing?