Diagnosis
The mechanical focus mechanism has worn or become uncalibrated, a known issue with some units after extensive use or temperature swings.
How to Fix It
There is no user-serviceable fix for a physically loose focus ring. You must contact Optoma support for repair or replacement under warranty.
If That Didn't Work
As a temporary workaround, once you achieve focus, avoid touching the projector and do not use the lens shift feature, as it can disturb the focus position.
Heads Up
Continually adjusting a loose focus ring can damage the internal lens elements, leading to permanent blur or misalignment.