One day I droped my Nerf Magnus and something broke, after tis the gun was not able anymore to hold the cylinder that sits on the spring back and I was not able to shoot. This is the thing that holds the cylinder back what gives the projectile its velocity.
I suggest to sand all sides smoth so that it slides better.
It could be that you have to sand a bit that ist fits and funktions.
Sorry for mistakes,
I am german ;)