Are you programming in Simpl or SystemBuilder?

If Programming in SB create a new page and create 8 new volume buttons with new joins(don't copy your existing buttons) and assign each pair of buttons vol + and vol- for each room. Also make a button on the main page that will allow you to flip to this page.
Once you have created the above build and upload then turn on all 4 zones. Go to the volume page just created and try controlling your volume for each zone. This should work. The problem has to do with composer and if you copied the created volume buttons there is likely hidden programming that you cannot trace and the only way to eliminate it is by drawing new buttons with new joins.

I was having the same problem with an Adagio system I was programming and I had to do a lot of wired SB programming to get around it.