medium speed isn't working on ceiling fan

there are a few reasons why this could be. Depending on what cap it has, part of the capacitor could be bad (I have a 5 wire, with a different microfarad for each speed) and one could be bad. Or something could be wrong with the windings.of the motor. You'll have to open it up to check these. For the capacitor You'll need to locate it and check it. For motor you need to locate the neutral wire for the fan, and the 3 for the speeds. Shut power off at the switch, and disconnect the power wires. Do a continuity check on all 3 speeds, going from neutral to low then to med, then to high. One of them will most likely read OL, or your capacitor will read out of tolerance. There is also the chance that your switch has gone bad. Set it to run on medium, and check voltage from medium speed wire from the switch to neutral/ground. If no voltage, replace the switch.