Unloader valve isn't working (probably)

If it has a "cut in-- cut out" switch, maybe adjusted,the points or switch needs replaced ? Does the motor hum like it's trying to turn the compressor?

If you use an extension cord or generator that can be the problem. Or as stated your unloader may not be working.
Or the check valve can be bad.

I have been using an extension cord, a 12 ga. I thought that might be heavy enough. I'll try it tomorrow without.
Thanks for the help.
Norm

Starting a 2hp compressor on an extension cord using 120v source is asking a lot. Does the motor have an option to allow it to run on 240v instead? If it did, I would use that option. I had a Sears 1hp compressor that would not start on an extension cord with the normal cut-in tank pressure. I finally got a new 2hp compressor using 240v. It will start on a 50' extension cord with even higher cut-in pressures. If you can't go with 240v, check your line voltage at the plug outlet to see what happens when it tries to start. Perhaps your house wiring will not handle the starting current. Charles Ranheim

Without an extension cord it runs great. Now all I have to do is move the compressor out of the shed to the house, plug it in
there and run the hose back to the shed. Awkward, but it should work.
Thanks for all the comments. Norm