I work in a chemical treatment plant in Oklahoma and we have the same issue with one of our clarifiers. We dont use alum for flocculation, we use FeCL3. Our issue is that our incoming solids concentration is too low. What is your incoming solids concentration?
Also, i would assume that you are conducting jar tests with each change?
How often are you wasting? If possible I would try to waste a lot less it sounds like you need to age your sludge. I would run MLSS test very often too and experiment with more lbs in the plant and less lbs in the plant to determine a control for lbs in your plant that runs the best settleability in your plant. I would also try to run the first cell with a lot lower air. You can use the first cell and run it anoxic or even anaerobic to cause luxury uptake to get rid of the phosphorus. I have experimented with that a lot in my plant and I found that have almost no air in the first basin dropped my phosphorus down tremendously. But naturally all the bacteria in your plant have phosphorus in them so settleability is huge. I would start by wasting a lot less if you are not getting a blanket.