I have a voice starting on beat 2 of a bar. In order to hide the rest in that voice on beat 1, I try to click the Starts Voice switch; but it instantly snaps back to the off position and the rest remains visible. However if I select the unwanted rest and choose Hide Rest from the Edit menu, I am successful at hiding the rest and the new voice on beat 2 has the Starts Voice switch turned on. Incidentally, if I then turn that switch off, the hidden rest does not show up, and the switch now behaves normally but seems to have no effect.
You could maybe add a screen capture, in order for us to help you. This could allow us to copy the problem you have and try to give you a solution
Did you end the previous instance of the (2nd) voice before the rest? If not, the start voice will not set, but the remove rests will set the start voice and retroactively set the end voice of the previous instance (which is why it is so convenient).
That must be it. Thanks.