Is it a new ARMS mount or an older one? If it is one of the new ones then it should work, if it's one of the older ones then that is probably where the problem is. The specs for the 1913 picatinny rail changed a few years back, that caused problems for a number of companies.
I know that Knight's & GG&G's rear sights work with our uppers, I haven't tried one of ARM's sights though.
Let me know if there's anything that I can help you with.