The aux fan is not supposed to come on with the compressor. It's purpose in life is to be a safety factor to prevent excessive high side pressure in the a/c system or excessive engine coolant temperature. It will have little to do with a/c cooling capacity except for the fact that it keeps the a/c from being destroyed which will indeed effect cooling.

As mentioned, the most likely problem is a low charge. Give us the pressures and we can help you diagnose it. The first step in troubleshotting a low performing a/c system is pressure readings.

Good luck,