Lower arm hang up

My lower spray arm hangs up ocassionally. When I tear it down, the rinse arm rubs against some type of flat piece of plastic (poor description, I know) that protrudes out of a horizontal slot in the discharge housing assembly. I can push the plastic piece back into the discharge housing but after time, it slips back out and stops the lower arm from spinning again. I have not torn into the discharge housing at this point. Does anyone know what this plastic disc is and how I can stop the lower arm from hanging up?