I have had more success just using POM/AT in a macro. But for some reason tonight, we were putting attempts on Lei Shen and even that macro wasn't working. I even tried physically clicking POM and then AT. I clicked them after getting a pyro proc, pyro'd twice, then clicked AT again to get two more pyros, but I didn't. I don't know what's going on with POM tonight. And I confirmed with another mage in my raid. He was having the same issue.
Edit: Discovered that it DOES work on Lei Shen, just not for an undetermined amount of time after the pull. Since Alter Time is only available for every other Combust, I just started doing my first Combust without Alter Time and instead using it for my second. Worked fine for the second. Never works on the first. Dunno why.
It doesn't work while hes shocking the raid before hes charging a pillar in my experiences. I think the debuff you have while hes doing his raid wide aoe is what breaks AT. Once I started to avoid that I stopped having problems.