STEMS not working consistently on DDJ-FLX4?

I just recently got an FLX4 controller and have been trying to use the stems feature to isolate vocals, instrumentals, and drums, but it doesn’t seem to work on every track. Some songs work fine, but with others I can’t isolate certain elements properly, or the quality sounds off.

Is this just because of how the tracks are produced, or is there something I should be checking in rekordbox? Are there certain types of songs that work better with stems than others?

Any advice would be really helpful, still learning!