Is it on a dimmer? If it is, be sure that the pull chain on the fan is on the 1st speed and the dimmer on high, then try to lower the speed with the dimmer. If that doesn't work, it's probably the connections in the fan. Sometimes the connections, like the screws can become loose. For this, you want to make sure the power is off. Take the fan down, check all the connections, might aswell make sure all the screws are tight and put the fan back up.