I have the same issue in the bath room wash basin on my 385GL 2005. The pipes is clean and clear but it will not drain properly when the boat is still in a harbour.
If the boat moves slightly, such as while on anchor, then it works perfectly.
The drain is not straight. Right under the basin there is a 90 degree angle. I assume that this causes an airlock, but I haven’t been able to figure out how to solve the issue