This is a known issue with iOS, described in this article:
http://www.imore.com/pissed-your-dictation-button-grayed-out-heres-fix-thats-only-slightly-less-annoyingApparently, when the system memory gets low, iOS disables the dictation button, starting with third party apps first. Closing some background apps and freeing up memory might re-enable it.
Unfortunately there's nothing we can do from the app side to prevent this from happening and it's something Apple will have to address in a OS update.