If the elodea is matted out and not actively growing as fast as it was, the Sonar will not be as effective. I agree with Shan that you need to give it more time to work.

Not to speak for Kelly Duffy, but in the past he has recommended burning back vegetation with a contact herbicide in conjunction with a Sonar app. The concept being that the vegetation will attempt to rapidly regrow, thus facilitating the uptake of the Sonar. This may be something to try if you don't start getting some results. Again, I don't know if this approach is applicable in your situation.