When you looked at this same helmet later, was it still not enhanced?
This happened to me many times, but when I looked at it later in the bag or on the hero, the enhancement had been performed. Which is still a bug, but at least the info may narrow down what is happening.
iPhone 4S 32G iOS 7.1.2 (11D257) RotK beta version - current as of date of post
I will put in a suggestion in the new boards for the official game regarding stones and failure rate. I know you agree. Basically, with the failure rate so high, I won't be spending gold on rare and epic enhancement stones.
If enhancing isn't even working, that makes is a definite no-no on spending gold for it.
iPhone 4S 32G iOS 7.1.2 (11D257) RotK beta version - current as of date of post