I am assuming that your compressor is oilfree type. Lets go over some basics to determine where fault is located. You will need to do some simple tests. If there are leaks the compressor may not reach set pressure. I use soapy water on all connections and on the head/headvalve (remove plastic covers to get to all parts that may leak) while compressor is running. Make repairs as necessary
. If still low pressure, while compressor is running and has reached max obtainable pressure (in your case 15 lbs), carefully place finger over intake port hole (remove filter). If the compressor is pumping normally but leaking there will be ample suction at intake and cause the compressor to speed-up when intake is obstructed with finger. If you feel plenty of suction and no visible
(soapy water) leaks, then the problem is probably the cylinder /ring is not correctly seating /scratched and air is leaking past cylinder. Make sure that the cylinder /piston ring is properly aligned and that the lip on the vinyl ring is up toward the head. On oilfree compressors you can examine the cylinder by pushing the piston all the way up, then look for scratches neer top of cylinder. If scratched install cylinder kit. If you do not feel suction but rather air blowing out of the intake, then the problem is the headvalve plate/reed valves are leaking. The reeds must seal perfectly on their seats and not be bent or have obstruction. Replace as necessary. Only one other thing to check and that is the intank check valve. Easy to check. Run compressor till you have about 15 + lbs in the tank then unplug compressor. Carefully loosen the inlet hose at checkvalve and listen for air leaking out of tank. If leaking replace checkvalve. Use caution when removing head to try and save the gaskets. If you see any model numbers on your compressor beginning with WL, you probably have a unit built by Campbell Hausfeld and can find parts at www.chpower.com
Good luck on your repair and email if you have other question.