Is it possible you arent seeing Improved Unarmed Strike apply until the end of your action queue? If you always end your queue under a polymorph that could be why.
As a separate issue & side note, I've not reported it before as it has been very, very inconsistent, but rarely I don't have the feat after the first polymorph into an air elemental, and have had to unshift and reshift for it to apply, though after this has happened during any playthrough I've done my darned best to try replicate it so I could report it properly but never been able to.