Loss of water to waterfall

We have an inground pool that has 3 valves - one that allows water to flow to the returns, one to the polaris, and one to the waterfall. After opening this year we discovered that opening the valve to the waterfall didn't allow much water to be diverted from the returns to the waterfall (in the past we just turned the valve to open slightly and we had a great flow of water.) If we turn off the pump, backwash the filter, and then turn the pump on again, everything is OK. Perhaps the backwash process "burps" air from the waterfall pipe? We"re looking for ideas as to what can be causing this. By the way, we noticed that the flow of water this year thru the skimmer is greatly reduced too. Tried the trick where you force water in reverse from the pump but this didn't seem to do anything.

I'd be grateful for any ideas!