Not trying to start anything here or beat a dead horse but you may want to check and see the recommendation for replacing strings/cables for your bow. I personally do mine every year (but I make my own). I see alot of people thinking that just because a string is expensive that it will last longer. Strings take a lot of abuse and should be replaced more often then I think some people realize.
Just thinking aloud here also but I wouldn' t think changing string brands would firm up the wall. My guess is that the previous string had stretched so you were actually underdrawing.