We ran into a similar issue. The boulders on the field were significantly firmer than the ones we got in the KOP even before they were used.
Our shooter's holding mechanism relies on compression and stall torque. The main flywheels create such a strong vacuum when they spin up that they physically suck the boulder out the intake unless we run the intake in reverse, stalling the motor to hold the boulder in place. Because the boulders were much, much firmer, the MiniCIM direct couple that is our intake was not strong enough to hold the boulder in place, causing the shots to fire prematurely before the wheels got up to speed, resulting in poor shots.
In our case, it was a fairly easy fix. We adjusted the spacing on the intake's wheels to be further out, reaching further on the circumference of the boulder and therefore not causing as much compression, allowing us to stall the boulder into place.
Image for reference