The stages on our Zeiss instruments are very versatile but they can have a habit of popping out of position - especially if someone bumps the lens against the slide ( please, never do that! ). It might not be obvious at first glance that the insert is loose but it is worth checking when you put your slide in. The bottom left corner is the bit to pay attention to.
It should be completely flush with the rest of the stage -
If not, it's easy enough to fix but first lower the objective lens to stop it from getting scratched. Take the insert out completely then push in the bottom left corner against the retaining springs. It can be a bit of a fiddle at first but the whole insert should fit in with a click and hold itself in place.