I have a Dolphin Diagnostic (one model below) and it has worked well for 3 years. I gave that one to my Dad and also ordered a Dynamic but haven't received it yet. I know there are different settings on that unit where it can do just the floor, floor and walls etc and can adjust the time. Also you may want to play with the top handle and the floats to see if that helps. I found if mine was on an angle it did the best job. I also found the best cleaning with both floats on the side of the cord.

Don't give up so easily, they really are a great product.
Joe