thats weird, I have never heard of Reward not working on duckweed. Are you sure its duckweed. there are several types of floating plants. one common problem I have when folks call is mis-identification of plants
with that said, you need to figure out how much water if flowing out of the pond. catch the water in a bucket and time it. lets say you get 5 gallons in 5 seconds. thats one gallon per second. mulitply that rate out to see how much water is flowing out of the pond in one day, then one week then one month.
one acre of water 3 feet deep is about 1 million gallons of water. figure out how much water is in your pond. once you know how much water is in the pond and how much is flowing out you know how long it takes for the pond to flush. Add recommended rates of Sonar or Avast a week or so before the pond is flushed.
Sonar does not last for 5 years, I dont have a label in front of me but I think its more like 18 months, provided the pond does not flush. Draining the water can can help but it depends on how much water flow you have. if you drain it down and it fills back up in 2 days I would not worry about it, wait for the dryest part of the year, treat, and hope it does not rain much.
