separate voices to practice

Hello I use different instruments to write my score so that I can play either the right or the left hand of my piano score.
I was able to download my score on my iPad, and using the three little lines and the mixer I could do just that
I am using now the same system, but the mixer does work any more and
now the two hands are always playing together, as if written with one piano stave
what is happening and how to make the mixer stay as it should ? thank you