It's possible that your desynthesis skill isn't high enough.. If you use the Desynth skill on it, and it opens up the window and tells you 'This item cannot be desynthesized'.. then it can! The fact that you can select the item in the window is the proof.
It's a bad translation/explanation of why you can't desynthesize it at that moment (too low of skill).. but if you right click on it, and the option is there to Desynth, then it can be :)