Springs, ammunition, and magazines.
A blowback pistol firing relatively low-powered ammunition is highly dependent on properly calibrated spring rates in both the hammer, recoil (return-to-battery), and magazine springs. If any of these has lost tension, reliability problems will result. Likewise, if the springs have been replaced, and are too stiff, problem s will arise.
Likewise, if the ammunition does not closely replicate whetever template the designers used with regards to bullet weight and shape, burn rate, and maximum pressure, reliability will suffer.
Finally, if the magazines are poorly designed, or have easily damaged feed lips or followers, you will have problems.