In researching this I found several causes including: Your control may be locked or disabled. You do not have "Allow Filters" turned on in your form. My reason was a real tricky one and not so obvious in the fact that my field was a SQL Server VARCHAR(400) which exceeded the MS Access Short Text size of 255. Apparently sorting and filtering text needs to be of the MS Access Short Text Data Type.
Who'd a thunk!:banghead:
