I would venture a guess that you may need to spend more time in Arban doing the triple tonguing studies. Practice them slowly; pretend you are first learning to multiple tongue. It may help also to go into the Clarke studies and practice the "k" syllable, by "koo"-ing entire excercises. Give yourself a rest when you feel fatigue or stiffness begin in your tongue. Play them piano! I have a student who is experienceing difficulty with the "k" portion of multiple tonguing, and have seen some improvement by sort of varying Mannys's "tooh" approach and saying "kooh" as in "koo koo". (she thinks I am anyway
).
As an aside- Manny- your Tooh approach and puppet excercise have been in use here with my students this week. What a difference all around! Helpful to ALL instruments (especially percussion)! Thanks from all of us here!
This post has been eddited ffor speling errers! And also to clarify a point or 2 so as to not kerfugeltate and befuddle any further readings.