I had a similar problem with Polaroid type 55 in a 545i holder.
After a big investigation, I discovered that the spigot that comes up to retain the metal clip on the film envelope was not holding it. The spigot was not perpendicular to the film and so the whole film envelope was escaping when I with drew the envelope to make the exposure, thus the film was truly unexposed which would appear similar to your situation. Yours would appear to be an intermitent problem, which is just as annoying and bad!!
Complete disassembly of the film back was fiddly (peeling back stickers to gain access and removing screws) and I tried to bend the spigot back to 90 deg and also increase tension, I inserted a piece of soft foam to act as an aditional spring. Sadly this didn't work successfully and so I am faced with getting a "new" 545i or giving up on Polaroid.
Suggest that you check the metal film clips from the ends of the exposed film and see if there are any witness marks where the film envelope has ridden over the spigot.