I had an extended spinning lesson Sunday afternoon. I, too, made the wheel "stick" when either foot would be all the way up or all the way down. Drat.
It turned out that, treadling on the shop's wheel, there was a lot of "stuff" in there. The instructor oiled the oil points and that made a big difference; then she showed me how to take the bobbin off and re-assemble the bobbin/whorl assembly and clean and oil it.
This second oiling really made a big difference in how easily I could treadle without getting "stuck."
So make sure the spinning wheel has all the necessary lubrication and then practice. I worked wearing my socks rather than socks + shoes, and I had to get my feet right up under the wheel to begin with. Depending on the particular machine, you might want to focus on treadling with most of the impetus coming from your toes or your heel rather than your "foot" as a whole.
Fun, isn't it?