The blastwave portion of flamestrike for fire has been bugged since the very first time the spells were merged in beta, yet not many noticed and many bug reports about it went unheeded.
Test it for yourself, go to a target dummy as fire spec and cast flamestrike. You will notice the blastwave debuff on the target.
Next, cast flamestrike on the target again, immediately upon the flamestrike cooldown finishing, and you will notice that this time the target dummy does NOT get the blastwave debuff and (if tested on real players) you will notice that the blastwave effect just does not apply (no snare).
It seems that the blastwave effect actually has its own internal cooldown, one which is longer than the flamestrike cooldown (and it is probably a hang over from back when the spells merged), and so it does not apply on every flamestrike. Instead, it just applies on flamestrikes where blastwave is off its internal CD.
Whether they will fix it? Nobody knows.
Does anyone even care? Nobody knows.
I wish it were fixed, though truly, mages have bigger issues than a flamestrike bug.
And this bring another question:
WHY THE FUCK DID THEY MERGE THOSE TWO SPELLS T_T ????