i would say that its probably an issue with the hight of the trigger bar on the left lock .
the trigger bar on the back trigger on SXS doesnt move as much as you think it does . so you will probably have to build up the trigger bar as storm rider stated , or get a new blank set of triggers and tune them for your locks .
ToW sells SXS trigger sets . but frankly building the bars up , isnt that hard .
the other thing you might look at is the sear on your old lock . compair it to the sear on your new lock . make sure they are the same and set to the same location at the full cock .
next make sure that when you place the lock into the mortice , that the trigger is all the way forwards . sometimes if the triger catchs back just alittle , when you set the lock in , the trigger bar will slip between the sears of the two locks