Had the same problem. Switched filter cartridges and both of my penguins spun great...... for about a week. Now they stopped spinning, again -.-
I Have em in a shrimp tank along with a sponge filter so im not too worried about filtration. Just going to let it be.. Might be a clogged filter if you havent changed it or your prop has dirt and gunk and stuff preventing it from producing enough flow.
It might also be a good time to check the little notches where the hweel axle sets. They can get build up in them which makes the wheel stick. They were one of the pains with Emperor filters which drove me away from them.