Maybe the shutter is off or the diaphragm is sticky. What battery are you using? FTb requires obsolete 1.35V batteries.I have used it MOST with my canon FTB. nice little spot meter. but here is the thing.
I can put a sheet of paper up, on a fence post in full light. take my meter reading on the left most edge of the paper, rotate the tripod so the spot meter circle is on the OTHER edge.
SECOND shot will have horrid over exposure/blown highlights. Even though the meter needle never changed between edges.
Ive shot perhaps 8 rolls of arista 400 iso 120 film... of the 4 that survived the developing process of learning roll film. THey were even worse.
I have only run 1 roll of shangai gp3 220.. i only had 5 images, the start and end, that came out clear/partial image.
I have more actual negatives on that roll of shanghai gp3, then those 4 rolls of arista iso 400 120 COMBINED
What 120 camera as you using? Clear or partial images are not normal and are likely due to camera malfunction or user error.