Back pressure? Not sure that has anything to do with it.
The recoil spring is calibrated to a certain mass bullet/slide/barrel system. The slide/barrel must have the same momentum as the bullet in equal and opposite directions, and the recoil spring is designed for that momentum. Throwing the suppressor on it increases substantially the amount of mass that must recoil, and the bullet needs to be higher mass to compensate with additional recoil energy to accelerate the slower mass against the spring to cycle completely.
Or go with a lighter spring.
Remember handguns aren't gas operated, they are modified Browning short recoil. They function due to mass/recoil/equal and opposite momentum, not gas pressure.