So it all comes down to this, its nothing fancy really.
If the stun says "Fully resisted" - Someone failed and put him on Dimishing returns.
If the stun says "+Stun", and he finishes the cast anyway - You didnt cast it fast enough.
This can be due to delay (which we have no way of seeing what our acutall delay to the server is, but my guess is that its quite high. Seeing as you sometimes get hit so damn far away from aoes its ridiculus.
My tip? Make certain you land the stun before more than half his cast is done, other wise its proned to not get registred by the server in time.
Been doing Ifrit bout 20-22 times, and ive had around 10 perfect stun-runs out of those, if im not 100% concentrated and instantly stun him when he cast, it will not register even tho it says "+Stun" well before the cast is finished (on my client, but not neccessarily on the server side.)
Well i hope this helps, and yes i finally got my weapon! \o/
*Edit: Worth noting is that if you're playing on the european servers. Well you are fucked. Because the European servers arent actually located in Europe. They are located in NA.