Radio Issues

I have been having problems with radio reception with my Digitrax system. I don't have a problem by myself, but others say they are having a problem when the group is together.
First I moved around the UR-91 receiver, which didn't seem to change things. I then put in another UR-91. When the next session came around I was told that the problems still were there. Most recently I have added aux power to the UP-5's, which is supposed to help. The next session coming up we will text that change. I made two sets of power which have seven UP-5's and one UR-91 on each string. Of course now one guy says the UR-91's should be powered separately. I'll try this other fix first.
The farthest any radio should have to reach is about 15 feet, well within what many other people are using for a distance.
I also bought some new rechargeable batteries that are supposed to be the greatest thing. They charge fast and are a higher voltage (9.6v), which makes Loconet happy.
I had a small amount of momentum on some of the engines, which I took out, because the operators didn't like it. I hope that wasn't the reason we though we were having radio problems. There are a few engines that are slow starters anyway and I hope that isn't confusing the issue. I need to ask more questions when something happens