Ah, good to hear that. Yes, with any sort of hop unit, you'll want to use the heaviest BBs that it will lift.
It may simply be an issue of BB quality and consistency. "6mm" BBs actually tend to be 5.95mm, and in practice vary from 5.92mm to 5.98mm when you measure them. It's more likely that heavier BBs will be made to better tolerances than cheaper, lighter 0.2g.
You may find that you can get better consistency from a tight-bore barrel, although finding or modify an existing aftermarket barrel to fit will be somewhat of a gamble.