If this model has a selector for small,medium,large loads. Does it have reset and far right side? If so make sure you are going to reset from a medium load to a large load.
Your questions about how is the water level determine. Your outer tub has a bafle with a clear hose connected to it. Has the water comes into the unit air is pushed up to the pressure switch #5 on the first diagram. when it gets to the desired level pressure swithes off then begins to wash. If all works ok sounds like you may have a faulty pressure switch. If you do decide to replace pressure switch make sure to push all the way on if not YOU WILL make a swiming pool out of your house.
Mike