It's a common problem with the kjw pistols.
The slide is too heavy and the red nozzles are really bad quality. Try to swap it out with an aftermarket piston if you can. If it's a blue nozzle it can still be the culprit.
To test for a nozzle crack, remove the blowback unit (the housing under the rear sight, and put the nozzle in the piston. With your thumb on the square opening if the nozzle and your index finger holding the loading nub, blow into the hole at the front of the nozzle. You should notice considerable resistance but not total blockage when blowing in. If you can move the bbu back out of the nozzle there is good compression otherwise if the air just flows out you may need to replace the mentioned parts.
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