John,
As a fisheries biologist and consultant, I have occasion to work with different materials to treat plants.
I have used Sonar to successfully eliminate coontail. It took 85 days.
Measuring the lake was critical, both surface acreage as well as average depth. As much as Sonar costs, we wanted to be sure the volume of water we were to treat.
Sonar was applied May 1 that year, and ends of coontail quickly turned pink. But, the plant didn't die in that 13 acre lake east of Dallas. I watched it, sweated, wringed my hands. (It was my first ever Sonar treatment) Weekly trips to the site, questions from the landowner we tempered with confidence from the Sonar rep who guided us through that first treatment. Finally, July 25, there was no trace of coontail at the surface, anywhere. It had starved itself to death, fallen to the bottom, where it was decomposing.
A big sigh of relief, and a healthy pond were the results.
So, if you have done your homework, measured the lake accurately, used the recommended amounts of Sonar, keep the faith.
If not, watch for Kelly Duffie's response to your answers. You may need to follow up with plan B.

Teach a man to grow fish...
He can teach to catch fish...