Usually when I start Notessimo it runs fine, but when I try to delete notes, they don’t highlight when I point at them, and when I click them, nothing happens. Why is this and what can I do to fix this?
Yep that happens to me loads of times. Unfortunately I don’t know why it is (maybe a runtime error or something) or how to get rid of it but sometimes playing the piece will reset the bug (but make sure you click precisely on the instrument you want to delete), and if that still doesn’t work try reloading the song you’re working on or reloading Notessimo completely.
Just make sure you don’t go around “deleting” all your instruments because even if they look still there they’re not when you play them.
If your playing full screen that could be the problem, try using a smaller window or editing your music on something like this
oh i figured this out a while ago. it only happens when the window is scrolled all the way to the bottom. so avoid doing that…
Really? Thanks a lot! 
Not always (although that definitely does trigger it)…in full screen it can also happen with (i think) the play only this sheet button, pausing it then editing…(either that or it just happens when near the end of the sheet in full screen mode or its just full screen mode)
If you click directly on (say a) bass drum 3 and it highlights and deletes a fender guitar instead, if you click off the sheet then back on the sheet…the bass drum is actually deleted and the fender is still there…
Also if you try to move something or click to edit a note while the glitch is active…the note is deleted (the highlighted ones that aren’t directly where your mouse is, are never affected after you click off the sheet then back on).
This glitch really pisses me off…as it happens often…which is why i am able to supply this information XD