Ask for less than double your current limit (not sure what you're asking for now). 2x or more your current limit and there's a separate approval process with Amex which likely decreases your chances.
Amex has not been doing line reallocations since 2008. Confirmed this about a month ago. If you're paying several times per cycle already, you probably have a good case. Card needs to be open at least six months before Amex will consider an increase.