Broken spring, bad piston o-ring, cracked piston head, broken nozzle, leaky nozzle, bad cylinder head o-ring, cracked or broken cylinder, cracked hopup unit, burr inside barrel, torn hopup rubber, partially stripped piston or sector gear that doesn't completely cycle the piston, jammed nozzle, clog in cylinder head, and any number of countless reasons could be the cause.
We really can't help you without more information. The best answer would be to bring it to a gun doc and have them check it out. Chances are you wont be able to find out whats wrong without opening the gun and having the necessary skills to diagnose the problem.
To check the compression you need to open the mechbox, judging from how you're talking about the problem I will wager a guess that you are not experienced with gun work. Bring it to a gun doc.