There should be two rollers, I find they get bunged up unless you set them in carefully and push them into position with the follower/spring assembly. But that's me. I'm overly careful with the magazines, as they seem to be the weak spot in the weapon's design.
Of 6 magazines, I've had one break, and two are currently on their way to breaking. They all seem to fail in the same place -- near the feed face (the round bit where the bullet come out. They've either failed on the face itself -- the first broke at the glue seam and snapped in half under pressure from the rounds, the second is cracking loose at the exact same place -- or they are cracking just below that.
I've taken to very carefully setting the mags in place with the catch pulled back. I don't load the bullets roughly. I suspect either its chintzy plastic, or the force of the bolt's operation placed high pressures on this part of the magazine, as it's inside the body of the gun. While the bolt should not be impacting the magazine other than to slide all the face and pick up the rounds, the inertia of the bolt might be getting translated to the round bit of magazine due to its position.
The only other thing I can think would reasonably explain this is because I have a loaded magazine in the weapon at all times, and over time the pressure causes the face to break loose. (Pretty friggin' bad design, if it is!)
I just wanted to see if anyone else is having problems with magazine failure. 50% seems awfully high, especially for the amount of rounds being put through it. (About 500-1000 before failure.) I would have expected to hear more about this, if the issue wasnt just particular to me, but the P90 is used by non-civilian forces. Militaries have a way of just chucking broken parts and buying new ones; they don't whinge about the quality of parts.
Perhaps my particular PS90 has the bolt hitting the magazine face especially hard.