I fly fish for SMB often and I find that when I have them chase but not commit I change my presentation to a stop and then go fast style. it seems to work best when they are waiting to ambush prey from behind rocks or logs in slight current, not out cruising in open water. I will let the fly drift and strip it slowly and then once I see them chase I strip it faster and give it erratic action, then slow down again. It doesn't always work, but works fairly often. Clay brings up a good point. change up your tactics and experiment, that is the fun of our sport