they are SUPPOSED to spin in a full circle, but the mechanisms break and rust really easily. there is a tiny metal spring on the end of two prongs, and when you squeeze the prongs the spring bends and spins the rotator full circle. however, if your spring is broken, it will only spin about half way if that. it is best to squeeze them more on the chest-belly than just the belly. if you open her up and look at the mechanism, and spin in manually, you can see that it indeed spins all the way when everything is working properly. The parts have a hard time resetting to "zero", or back to the beginning position, and therefor don't spin all the way.
I experimented with many many ways to fix my Tabby, however attaching a new metal spring to the mechanism proved almost impossible. i still havent completely figured out a way to do it yet. the best you can do is open them up, check to see if the spring is broken or not. if it is, there's probably no fixing it. if it isn't, you can probably clean it up and help it reset back to zero and check if that helps.