Monti, it seems as if you have a tension issue, there is a few things you can do to try to solve them. Remove the upper and lower clamp screws, and check out the little pivot pins that are in the end of them. Make sure the pins can move, and sand the end of the pins to scuff them up. Doing this should make the clamps hold on to the blade better, so it can't slip in the clamp. Another thing that should help solve your issue, is to apply pressure to the upper arm when installing the blade. What I do when I install a blade, I always attach the lower end of the blade in the clamp first. I make sure that there is enough blade under the clamp screw, then for the top clamp, I always apply downward pressure to the upper arm while tightening the clamp screw. Not a lot of pressure is needed, but, if done right you'll never have to adjust the tension screw again. I've had my EX21 for 5-6 years now, and I only ever needed to touch the back tension knob for spiral blades.
Len