The other day I struggled to get a pair of Polish-made Stomil Antilopes, dating from the 1980s, off and back on a 20" wheel, while my others have been no problem. I suspect having spent the last 25 years in a basement had something to do with it
Continuing on the subject of Stomil tires, a bicycle tire brand that sunk without trace in 2005, I had two bikes that had barely moved since then with some of the last stock fitted, but each with different tread patterns. One pair were soft and came off easily, the other pair were like plastic and a nightmare. However, after some riding, the plastic-like ones became easy to remove and refit too.
There are a lot of factors that can affect tire removal and replacement.