It may has a condition that your pistol cannot feed the BB bullet due to lightweight gas supply. You just simple remove a spacer to solve the issue. Alternatively, you may consult your gunsmith for advancing fitting to solve this issue as well.
Increasing in FPS can be achieved; however, it may require other parts to collaborate and advanced skill is needed. improper installation / fitting can shorten the life of the product.
*PS: Please note: installing parts / assemble /fitting airsoft gun will need intermediate knowledge; for expert or airsoft gunsmith.