Hmmm. Well, you say the back is in good condition. So, there are no issues with the seals you can see. What about the seal around the dark slide slot (which is inside the back)? If the slide was left out for a while and some light got in, and then you advanced the film through the rest of the frames, you might get a leak near the start of the roll without seeing it on the rest of the frames. Just a guess... if you have to reseal the back, it's very easy to do on the older Pro and Pro-S backs (I've never used a Pro-SD).