There's definitely debris caught in there. Sheer Descents need to have a rock trap in line and also they should have a small filter as well.

SD has a cleaning tool much like a thin blade that can sometimes clear the end. The SD would need to running to do this so you will get wet.

Organic materials like leaves, will eventually decay. Small pebbles or similar will not. The small water passage ways inside are not designed to allow debris to pass through.

An air compressor on the return side of the pump to the SD may purge any debris??? Cutting the pipe on the return side and use a shop vac to pull back debris??

One would have to be creative to make an adapter of sorts to fit over the spill lip to push debris back towards the pump.