Reefer,
1. How long did you leave the Levamisole in the tank before you did the water change? My limited experience with this (3 consecutive weeks) is that Levamisole did seem to take a little longer to kill them than Flatworm Exit (2 years ago). Since I didn't have a "bad" infestation, I didn't do my water change or start running the carbon until 48 hours after I dosed.
2. If you used Flatworm Exit a few weeks ago, you probably got rid of the bulk of your population then, hence the "blurred water." Sebsequent treatments will be killing those that the first treatment didn't get to (possibly deep in the rocks) or the first generation's babies.