Sorry, no band adjust ment on a THM400. If you pull the pan, it will tell the whole story. If it looks like a science project, time for a tear down and rebuild. You still may have a pressure problem which it's going to take a person with good detailed experience to find out why. If you're fortunate, the main pump spring is weak and can solve the problem. I doubt it though. Accumulation of material usually causes lip seal bypass not allowing pressures do apply the clutches. If the shifts have been deteriorating over the past while, build it, it's time.