There is a very well known and well-documented bug that came with the 9.1 patch back in June. It affects all store-bought mogs as well as the RAF ones. You still own them, but they will pop in and out of availability for transmog without rhyme or reason. It may be there one day, gone the next and return four days later.
Regardless of what some folks have said, there is not a known fix at this time. What fixes have been claimed are mostly just good luck of it being a phase of them coming back to be available.
The staff are aware of the issue and they are working to resolve it as soon as possible. It’s just difficult because it isn’t affecting everyone, and from what was said by the last time a Blue checked in, they’re having some difficulty replicating the issue.