All of a sudden I've started having problems with range in my Pro 500 collar. I get maybe 50 ft of range total. Anybody dealt with this problem before. And I just changed batteries in both transmitter and collar. Ado the 50 ft is line of site, so I know it's not due to walls, thick brush, etc. even so with all of these factors my collars should get several hundred yards at least. Any help please