Why does my fan only turn on at maximum power?

Why does my fan only turn on at maximum power? Hello. I have a standing fan next to my bed. When I try to turn it on at minium, it slowly begins to spin (I mean really slowly like 20-30 rpm) sometimes after a few minutes it gets up to speed, but most of the time it doesn’t start before I crank it. When its up to speed I can then turn it down to my desired level. The problem is when this trick doesn’t work, and even though I crank it to max it doesn’t speed up. Any ideas?