Why are you doing a recall when there are other dogs sitting behind you? My recalls are typically with no one behind me. AKC type recalls would be done with a group but they were lots shorter than our 30 pace recalls. On top of that, I'd change up two ball in two respects -- first I'd go to throwing right then left instead of behind me, second I'd do some recall practice with each session, too.
Usually, when something starts screwing up reverting to "what works" or what was working is a good step, too.