HELP Problem with RB67 Polaroid Backs

I got this polaroid back with my RB67 for Christmas. I took a few photos and every one came out with the print developed like this. We opened up the back and found that, on one side, the clasp meant to hold the rollers down was snapped and this meant the rollers weren't compressing at that side (one of the FP100C pouches would be burst and spread normally but the other would remain unburst).

We did some supergluing and that fixed it - but I also bought a new polaroid back just to be safe.

After Christmas I tried with the new back but this time the print pulled out of the back with neither pouch burst. I've repeated this many times (developing them 'manually' by rolling over them in the dark bathroom with a wine bottle), with the new rollers in both polaroid backs to no avail.

These rollers look completely new - no part is broken; for some reason the rollers just aren't bursting the pouches. The only thing I can think of is that the top roller is able to move slightly but I feel like this doesn't explain it totally.

Every pack-film polaroid back and camera I own seems to use the same roller assembly, which pops in and out for cleaning. I'd try swapping a few out, or see if you can find a dirt cheap land camera or back? I've got something like 6 different pack film cameras or backs, and never had an uneven (or no) pressure problem.