Water Not Draining From Sink

When any water is in the sink it won't drain away easily, there is no blockage in the pipe - could it be trapped air? If so, how do I rectify it?
Thanks in advance.


We have had this problem before. To rectify this, we first flushed and cleaned all the pipes. We then made sure that when we levelled up the caravan, we had it slightly lower on the drain plug side. This did help but you have to bear in mind that the actual drain pipes do have a narrow gauge.