First suspect for me would be piston seal especially if it has not been used for a while, then breech seal, then spring - springs usually die slowly unless they snap or are left loaded for long periods (gun cocked)- take it all to bits and evaluate.

The cheap mod5 I bought for this comp. really had only the remains of the piston seal left and what was left was like cheese, a seal from chambers and an hours work and it was like new.

Best cheers
George