Shark Rocket Duoclean HV380 soft roll won’t spin under load

Shark rocket duoclean soft roll won’t spin. If I take the soft roll out, I can see the “gear” that it snaps into is spinning, but when I put the soft roll in and snap it back in, it stops. I figure it must have some sort of “slip” designed in so if there’s too much load on it, the belt is allowed to slip instead of snapping or burning out motor. I’ve disassembled the vacuum head to check the drive belts and found the belt pulley does indeed spin relative to its axel (which means axel spinning doesn’t necessarily spin the brush). There was also a lot of black dust around the pulley and on its tiny splines. Is there an insert that should go on the axel to make the pulley spin with it under higher load? Without the soft roll spinning, this vacuum doesn’t pick up anything on hard floors. When it works, it’s the best vacuum I’ve ever used (including Dyson).