New washing machine not draining.

So I have recently had a new kitchen fitted which has a larger sink and therefore the waste pipe has a shallower runaway angle after the U bend. my washing machine was connected up and worked fine then stopped working which I thought was a faulty machine so replaced it as it was very old. the new one however has the same draining problem. when I disconnect the hose from the U bend it drains into a bucket no problem so the pump is working fine. the U bend has been taken off and inspected for blockages which there aren't. Why will it not drain when connected to U bend?