Sometimes when I turn my kitchenaid stand mixer on

sometimes when I turn my kitchenaid stand mixer on it won't move, like its slipping, but if i give it a gentle push it runs. it doesn't do it all the time. What could cause this? It looks like when you turn these off, they have to "seat" to be ready to run again. If I look at it, when it doesn't seat at turn off, it will slip when you turn it on. Anyone have an idea of what could cause this...or could it be oil on something? Thanks