is there any way something has got into the pvc
pipe and is blocking it? a snail got into mine!! it went up and over my overflow box through the syphon pipe up and over the other overflow box and into my drain pipe.
1.5" pipe can take 2100 gph
on a gravity feed so pipe size is ok as your pump is rated at 1900 gph
if the return pipe is clear then I'm afraid I'm at a loss and can only suggest you put a tee piece in the 1" flexible pipe and plumb it to the sump inlet with a valve on it,this will allow you to adjust the water flow to the DT.
Unless you want to run your drain pipe into a bucket and calculate the flowrate coming from your DT just to be sure its not blocked.