What sort of movements are you using when agitating the tank? I ask because if the agitation pattern doesn't essentially cause the liquid to "tumble" it may be that you are essentially agitating the fluid at the top and bottom of the tank, while leaving the fluid at the centre relatively untouched.

In addition, your problems with the Paterson tanks may be due to the same issue - the Paterson tanks and reels work well for a huge number of people.