Well,
If the OP really wants to get to the bottom of this problem, only one variable should be changed every time a film is processed.
If a different film developed as expected, then the developer is likely fine.
If the negative is really faint, one or more of four things happened, and we eliminated the developer, so that really makes it three things:
1. The camera does not expose the film correctly. Maybe the automatic settings in the camera doesn't work below ISO 100? I don't know. Just thinking out loud.
2. The film was laying around too long and its latent image did not retain its strength in the developing process. I think we can not say for certain that this is true, because if there were months between the first and the last frame being exposed, then the problem should be worst on the first frame, and least bad on the last frame.
3. Somehow the developer got contaminated prior to, or during the process, or something else happened that drastically decreased its activity.
I don't know what else it could be. If it were me, I'd put another roll of Pan-F+ in the same camera, but shoot the roll quickly and develop immediately.
The whole latent image thing is weird, considering I have developed film that was exposed in the 1960's that still came out OK. I can't believe that a couple of months will make that much difference!