More information would be needed to diagnose your problem more accurately, but here are some tips to start:
1) Fans will not work with dimmers. If you have a dimmer controlling your fan, it will only work when the dimmer in is the full intensity position (if at all). You will need to replace the dimmer with a fan speed control. If one switch controls both the fan and the light, this will not solve your problem, as a light will not work with a fan speed control switch. You will have to settle with a standard single pole switch, a remote switch, or get one more wire between the switch and the fan.
2) If you have a fan speed control switch, but the fan doesn't seem to respond as it should, you may want to pull on the pullchain directly on the fan. If the pullchain is not set to high, your speed control will not work correctly.
3) If #2 or #3 don't apply to you, and you are simply using the pull chain on the fan, you may have a bad switch inside the fan housing, loose or disconnected wires to the fan (unlikely), or simply a bad fan motor.