It's not the best ammo and it's pretty nasty stuff. Sigs tend to be really sensitive with power factor, it's entirely possible the Monarch is underpowered, but make sure you are running the Sig well lubricated and you are getting your grip high enough on the gun, it takes a fair amount of cut back to not cycle.
I've shot thousands of rounds of Monarch through all of my Sigs (P6, P228, P226 .40) and never had one issue. I've shot them all in USPSA matches and almost every week I'm the only shooter that doesn't have a single failure, including Glock shooters. My guns are very well lubricated but I only clean every thousand or so. Like I said, 100% reliable for me, or damn close to it.