I use that brand and it has been dissolving. Not the best AMS I have used. It does appear to have poor quality control as you can see the larger size particles in the bag that  you get when you buy the AMS that is used in fertilizer spreaders. The bigger particles are 1/8 inch diameter or a little bigger and kind of hollow looking. They are not what you associate with spray grade AMS.  Maybe your bag had too many of the big particles?

  Another issue is when I get a pallet of the stuff there is always a bag or 2 that doesn't get sealed properly and it is leaking out and making a mess all over in the trailer.