[Zubersoft]

It sounds like you may have performance mode enabled. On the main library screen, there is a sheet music stand icon at the bottom right. If it's orange, tap it to clear it. Then you should be able to zoom again. If performance mode is enabled, them I'm not sure why you wouldn't be able to zoom. Zooming is not supported with the two page display or the half page display, so if you are using either of those, that would explain why you can't zoom.