I did the same thing you did Solomance (replace the nozzle and tappet plate) to my P90 a while ago, except I also replaced the motor for a Lonex A1 Titan. Still ran into the same issues.
I found that when I use my Echo1 magazines (fully filled) which have a weaker spring in them they feed fine, however when I use my Tokyo Marui magazines (fully filled) which are newer and have a stronger spring, I run into feeding issues until the 10th-20th shot.
Here is hoping the Sector Clips will solve your problem.
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