Practice approach

I have two questions:

Concerning practice, Ive been working examples and often get them to what I would call an 85% level.  Then I plateau.  Do you recommend we work the exercises until they are played perfectly or do you suggest we get to a very good level and then cycle back periodically?

Second, my draw tones are still weak and sound pinched particulary on 4 draw.  Does this take time or should I examine my technique?  Of course I recall when I first started and could not play a 2 draw at all...I sent the Harp back to Hohner complaining that it was broken.

Thanks