Hi Paul,
This shouldn't happen as there is no restriction other than your device storage. Given that set lists are very small text files, I'd expect that this should never be a reason on a modern device such as yours.
Can you give me some more info:
What happens when you click on Save for your set lists? Remember that if you edit the existing current set and just hit the save button at the top of the set menu, it will save on top of the last loaded set. You have to click on the + icon on the set menu and select Manage sets (you can also get here using the normal settings menu) if you need a new set name. Do you see any warnings or errors when you try to save? I'm assuming the newest set list just isn't saved at all? Can you try to save a set again with a new name such as Gareth test and then send me a copy of your OpenSong/Settings/fileWriteActivity.txt file to see if I can see any issues. Also, if you have a crashLog.txt file in the same folder, send that to me as well.
If your device has updated the OS, occasionally you need to re-establish storage permissions (which could be a reason for a file not being able to be created/saved). You can re-establish this by going to Settings > Storage > Local storage > OpenSong (ROOT) > Select storage location. Click on the 'Select storage button' and navigate to the existing OpenSong/ folder (remember to use the ROOT OpenSong/ directory ,NOT the OpenSong/Songs/ directory). Allow permission for the folder and click on OK to boot the app again (it will force a fresh indexing of your songs again).
Best wishes,
Gareth