If the extension jambs stick into the room, they can be planed off with a hand or power plane or with a router and modified base. For extension jambs that are too small, pulling the window in works sometimes. Gluing on a strip to the extension jambs may work, also. I have done both. Sometimes it is the sheetrocker's fault, where they do not use enough screws. I have added screws to pull in the sheetrock and I have shaved off a layer of sheetrock in other cases.
The extension jamb battle is always the nightmare for trim carpenters.