How do I adjust the gap on interior French doors?
Did you check the frame again after hanging the doors?  Could be that the frame wasn't securely attached to the studs and the weight of the doors is causing the frame to pull out slightly.

Also, maybe too late for this now, but you mentioned that the old frame was about an inch wider than the new one.  That seems a bit strange.  Is that the rough in dimensions, or the finished frame dimensions?  Usually the rough in is a couple inches or so wider than the actual door frame dimensions to give you some room to plumb things up with shims.

So if you had a 48" door set, the rough in width would be 50" - 48 for the door, 1 more for the frame, and 1 more for shimming.