You most probably have a partial clog somewhere that is restricting flow enough so that the hose gets sucked flat.
Take a garden hose and try to blow through the hose backwards. I assume only one end is accessible? If not then blow through the normal way.
If you have to do it backwards, put a towel on the drain so you donít blow nasty water
Iím thinking a hair/soap scum clog.
Usually the pump doesnít connect directly, usually use I believe is to drain to a bucket and have the pump in that bucket.
Mine is a diaphragm
pump, those can be mounted up high, out of the bucket so itís easy to get to for service
Be sure to have a strainer before the pump.