Hello.. As RR01 says if it's a new pistol take or send it back to the shop you bought it from and get a refund or replacement, if it's still under warranty..
If it's not new or out of warranty, first things first, take the slide off and inspect the blowback unit loading nozzle and the blowback unit housing, make sure nothing is loose or cracked causing the slide to stick, and check the blowback unit housing screw make sure it's not loose or damaged because if it's loose or damaged the hammer can get caught on it..
ATB Marc