The pc9 uses bx width parts so You'll have to use their sear and disconnector or a stock 10/22 sear and disconnector.
Or you can just use the hammer from the kit
The firing pin works off inertia the spring keeps the pin to the rear much like a 1911 without the spring it won't stick out enough for the hammer to hit with enough force to work
The long plug has a shoulder for the spring in the same place it would be on a five inch gun. Using the standard plug you essentially reduced the spring rate
Did you look at a 3inch version next to a 3.5 ? Even the browning sales adds mentions the receivers being dimensional the same. When I bought my 3 inch I noticed no difference in the size of the loading port .
I thought I had the same problem and was using a little one shot dry lube but later found out the drop tube was binding turned it a quarter turn and been running fine since