Most likely it's because you accidentally unscrewed part of the zoom eyepiece assembly while removing it from its case.
Here is the eyepiece case.
Next after the top of the case is removed.
The eyepiece should screw off, leaving the case bottom.
Instead, it may screw off and look like this.
If this has happened, then this ring has come unscrewed off the eyepiece and has stayed with the case bottom.
The eyepiece without the ring will look like this.
Remove the ring from the case bottom and screw it back on the eyepiece.
Here is the correct appearance of the complete eyepiece.
To prevent this from occurring again, when putting the eyepiece in its case don’t tighten it down as firmly. This way, the ring should come out with the rest of the eyepiece instead of staying with the case.