Water not flowing fully away from external drain

[3wrlcs]

I am having problems with water flowing away from an external gulley trap. This isn’t as simple as a blockage but seems more like an air lock. The trap is clear and I have had one of those flexible rods with a spring on the end down it and that travels 6 or 8 feet without obstruction. This is far enough for the drain to reach the main drain. If slow water runs into the drain e.g. sink flow then the drain fills and overflows. If I put a pressure washer down to the bottom and fire at the back wall there is a rush of water away from the trap and the high pressure water will continue to flow away without issue. In normal use the drain will refill and slowly empty but will overflow with greater flow into it. This drain is connected to a garage sink, washing machine and roof drainage and is only a few months old and doesn’t have fat down it. It is also the last drain joining the main house drain and all others flow without problems so the main drain isn’t blocked. Any idea what could be happening in a case like this?

 

[Hugh Jaleak]

Is the pipe away from this drain laid to the correct fall with no dips or high spots? I'd also be concerned that there isn't a partial blockage of cement or plaster type material in there, spring may ride over it, you need a 4" plunger on the rods or ideally a CCTV inspection I think to see whats there.