Yes, it's normal for the GPR to have nipp problems. Usually the "hammer fall" problem is from OVER TIGHTENING the lock side screw!
Black Powder rifles aren't cannons,,it's just a simple gun. After cleaning, the lock should fit in it's mortise with just thumb pressure. Then the side screw set just too hold the lock in place(inch-pounds of torque) and no more. If you tighten that screw really snug,,,then the hammer is forced too the side,it'll never hit any nipp proper then!
It's a gun,,so every thing needs to be really tight right??(wrong),.I'd almost ask what "hot shot" nipp.? but it doesn't matter. Theres about 4-5 that make after market nipps for the Lyman/Gp models,,Knights' "red-hot" in 6mmx.75mm works great. I've been there/here,,done it,,know it..seen it 100 tymes. Back the screw off the lock.,,,,,,,,,,
luck to ya,