Dishwasher only drains in specific circumstances

Everything appears to work Ill start a cycle and it will wash the dishes fine the problem is when it comes time to drain it drains only a little of the water out. I replaced the drain pump a few days ago because it wasn’t draining at all no matter what I did.

However If I turn the tap off so no water is supplied and then start a cycle it will drain the remaining water out of the system no problem so the pump can do its job that doesn’t appear to be the problem, but during the cycle it just doesn’t seem to want to drain properly.

On the red line there is a little black flap that opens and closes, sometimes the water will go back into the dishwasher and the flap wont stop the water I am unsure if its supposed to back-flow like that. Either way I don’t really know what the issue is, to me it seems like it just doesn’t get to a point where it will start to siphon the water, as when it does drain it will siphon correctly and I I can see it working as intended, its just during a wash cycle it doesn’t really siphon any of the water out, its like the pump pushes the water up the hose initially but then it wont continue to siphon the water out.

I hope someone has a good answer as I cant find anything close to the issue I seem to be having.

Thanks

LG ld-1420t2