I know exactly the feeling of moving the tongue and nothing... I think when we get caught up in mechanics we lose the music side. Lots of good advice here. IF you want to get the speed up do the tried and true metronome at a slow speed ..play it perfect then move it up a notch or two.... everyday. It's a whole lot easier to track your progress.
Chuck's idea is pretty good .. it will help you get the feel
I use Ah -EE ... that's a good combo of tongue and air.
and lastly
some horns are easier to play them on .. I was playing a student model Coueson the other day that just flipped and locked partials so much easier than my Bach Strad ... so I dumped my Strad..kidding I'm KIDDING
good luck... it's frustration... but very cool when you get it!