Have you tried opening it while it's running to see if the arms are
spinning?

Is there stuff caught at the bottom? There's a grinder there that's
supposed to dispose of food particles by grinding them up. Perhaps
it's not working or it's clogged.

You shouldn't need to pre-rinse, although I've found that if I have
rice on the dishes it sticks to them in the dishwasher.

I presume you're not putting utensils in such a way as to impede the
soap door from opening.