When you say tries to run, do you mean the motor is being powered but fails to spin?

If it is, then it sounds like the unloader valve isn't unloading, so the compressor is trying to start under load and failing. On the pipe from the compressor to the tank, there is a one way valve (normally at the tank end) which should only allow air into the tank. Somewhere between the compressor and that valve, there will be a small pipe T'd of it, which usually goes to the pressure switch. When the compressor reaches the set pressure, as well as the motor being stopped, it should also open the unloading valve, and dump any pressure between the compressor and the one way valve, so there is no load on the compressor the next time it starts.