400x does not seem to work at all which others are just fine

Hey everyone, I’m using a microscope from my college and I’m having an annoying issue. Every other setting (40x, 100x, and 1000x) work perfectly and I can switch and fine tune them just fine and see perfectly. But once I switch to 400x I see absolutely nothing. I’m observing an elodea plant and the 400x is just a green blur. I’ve cleaned the lens multiple times and tried messing with the light but doesn’t help. No matter how slowly I turn the fine adjustment knob nothing appears. Am I supposed to be doing something else? Every other slide has this same issue with everything but 400x. It’s a Motic microscope.