The source of our problem lies in the two yellow highlights.

The rectangle highlights the underside of the hinge well of the ribbon cover door. You should be able to see a small spring through the hole.

That spring should be attached through the "eye" highlighted by the yellow circle. Unfortunately this seems to wear through with friction and causes our fault.

Repair the split as best you can. I have been able to get away with a drop of super-glue; you may find that you need to melt it together if the split is slightly larger.