Had the same problem with mine.
But a friend at the camera shop showed me that if you remove the little cover on the lens sleeve you can get access to the screws underneath. They need tightening when the rear mount is back in place. When tightened put the small cover back in the slot and like new !!
The sleeve rotates round, and there are three screws from my recollection of the last time it happened.
Simple job, takes about 5 - 10 minutes, but baffled me too until I was shown what the problem is.
Hope you get some resolution.
Steve
Thanks for confirming what I was trying to find! Mine were firmly in place to the point I could not loosen them.
Laura - below are some quick shots from mine. If you do this, please make sure to use JIS screwdrivers. The small screws on the barrel are very easy to round out if not using the right driver. Lesson learned in my early days of DIY lens work
