I thought your boombox had properly working belts, but now, I think you simply need a full service, which includes new belts and proper cleaning, and yes, definitely clean the pinch roller with isopropyl, the strongest ones you can get (100% vs 70%). Sometimes, the roller even needs to be lightly sanded to bring out a fresh surface if highly oxidized or hardened, but replacement is the best. You only have a few units so the cost is well worth it. For those folks who have scores, even hundreds of boomboxes, it's probably not worth it to do that to all their units. it simply sounds like your tapes are slipping, some worse than others probably because of the friction coefficient of the tapes vary, or because longer tapes are more difficult to pull than shorter tapes. I've seen tapes from goodwill have so much friction that they don't play at all. Press play and the friction of the tape sandwiched between the head and felt pad beneath the tape stops it dead cold.
As for your experience, you need to understand that if you are going to be buying 40 year old electronics, especially those with mechanical parts such as cassettes, these sets definitely require periodic maintenance to run their best. Even if a unit is advertised as "all-working", it doesn't necessarily mean "perfectly-working" since it begins to deteriorate the moment it leaves the factory. Your best bet when buying units that are "all working" are to see if the seller advertises specifically and exactly if and what has been done to it. If it says belts changed, then at least you know that the belts are new, but it doesn't say if it had proper cleaning, pinch roller refresh, lubrication, etc. Also, does it say controls were cleaned? If not, you can almost expect a certain amount of static in the controls when operated.