You should first check for any obstructions in water vents located in the center or back of the pan (where you water not draining). If you have a coffee straw, try poking in those vent holes to clear any unseen obstruction. Next, if you have a sink next to the dishwasher check that it's draining too, sometimes it is the reason as they are often connected. Turning on the water and garbage disposal can often clear the obstruction. If all else fails, pour a little Draino in the pan and wait.