Can you tell me how you've assigned your channels?
In one of my earlier bands the main mixer used by the band was fine until we decided to mic the drums. Then we never had enough channels.
I purchased my own mixer and would do a separate drum mix with my board and then run that signal into two channels of the main board (since we were running system in stereo) using XLR cabling. This allowed my mix to be mastered similar to having extra channels on the main board but in fact only using two channels of the master console.
I could also control my master out and since I used a complete rack system, (mini PA see below) I could run a separate stage mix and volume control separate from the main mix.
I don't know if that makes sense or not but the drums were grouped as a separate mix on the secondary mixer.
As it were, I even incorporated my electronic drums easily into this same situation. The biggest difference was my electronic drums had the sound module assistance to control the drum mix so I only needed two channels out and then into my board simply for the sake of a more controlled mix to the master console and the independant ability to use my stage monitors which I really needed with the electronic kit or there was NO stage volume at all.
If I've confused you worse, sorry. If you have more questions, I'll see if I can answer them.