Is the button initially disabled at the start of play? I was trying to loop from 0 (the start) and noticed the button seems non-responsive until the time display increments, slightly. With much work I determined you can actually loop at almost the start but its frustrating. I was trying to loop the start of Vltava (Die Moldau) to listen carefully to the start of the flutes but unsuccessful.
After much experimentation I figured out I could use the spacebar to start play and then quickly hit it again for pause and get an active button.
Perhaps there's a delay of 1ms for the code to reach button availability/activation once play starts and this could be fixed by making the A-B loop button immediately available when the play button is available.