Why won't my film advance?

I have a Pentax Spotmatic. My dad got it in Japan in 1969 and used it a LOT up until maybe 10 years ago when he moved on to digital. Anyhow, it’s a great camera. It’s taken a lot of beautiful pictures. He gave it to me as a graduation present last year, along with a few lenses and a flash. I love this camera. I’m pretty new to film, and I still use my digital a lot, but the picture’s I’ve taken with it have always turned out well.

ANYHOW. Lately, when I put the film in, before I close the door I get the film going and advance it 3 or 4 times. Everything always looks fine and the film is winding perfectly. So I close the door, advance it a few times, and start taking pictures. Then it becomes hard to advance the film for maybe 2 or 3 shots. After that, it feels normal. But when I get to the end of the roll (or so I think), and try to rewind it and take it out, it always becomes apparent that the film never advanced anymore. So, all the pictures I’ve taken were never really taken. The film stays still, basically. I can tell because when I go to wind it, I only have to turn that little nob a few times and it’s totally rewound.

Does anyone have any idea what this could be? I don’t want to get rid of this camera but I’ve taken quite a few rolls of film now where this has happened. It was happening a lot a few months back, then it stopped, but now it’s constant. I’ve taken it to be looked at and no one has been able to help, so I thought I’d give you brilliant people a shot.