PC fan not spinning but says it is?

My case fan is acting really strangely. I have a fan voltage regulator and when I turn it on it will spin really weakly for about 5 seconds then stop. The controller (NZXT Sentry 2) says it is spinning though. If I constantly change the voltage (90%->100%->90%->100%...) it will keep spinning but not as strongly as it should. I will randomly twitch like it's trying to spin and sort of go weakly for a couple seconds sometimes but never maintains a constant speed. I know it's not the controller because I tried it on another fan and it worked fine, so what's the problem?