I think that's pretty common. The spring (standard type) has to be the perfect length and they are pretty weak at the end. Our Coyote Cap CB 97s were designed from the factory to hold 6 but won't hold 6 Remington shells. A small amount of gunk or leaked shot will keep them from pushing out the last one.
I make magazine springs that have the coils alternate back and forth from standard width to a slightly smaller width that will fit inside the wider coils. This makes it so you can have a longer spring that compresses shorter than a standard spring. We also have this type of spring in all our 73s. We've found that magazine springs (especially light ones from one of the top SASS gunsmiths) often get weaker over time and do not push out the last rounds when dirty. This has messed up a few important matches. Also, if you put a magazine tube liner in a 16 inch trapper 73 you will never get a standard spring to work to hold 10 rounds. I like to have springs extra long so there's always plenty of force.
I've thought about making these available for sale. I don't have much free time but if there were enough interest I think I would make up a batch.