Gully drain is not emptying

I had my back garden tiled over and a gully drain was installed as per the attached picture. The drain won't empty - I assumed this was because of a blockage, however, I manually emptied the drain and it filled up again about 15 minutes later (the kitchen sink was emptied much the same time so possibly this is connected). I also put a hose down the drain and didn't encounter any obvious blockage.

My worry is that the builder got the drainage levels wrong and the gully drain is too low, meaning all the work needs to be pulled up to fix the problem. I'd be grateful for any views on the issue?