Why wont dishwasher drain?

I bought a dishwasher brand new a few years ago. The first time I used it, it wouldn't drain. At first I thought it was because my water pressure was low, but have since found that that is not the reason. Can anyone give me any suggestions as to why it won't drain? it is a portable that rolls across the floor to the faucet and hooks up. It seems to wash ok, but doesn't drain anything. So needless to say all the water stays in the dishwasher. I have never been able to use it. Can someone help?