My guess, without looking at it, when you are setting the rear trigger, you are lifting the front trigger enough to trip the lock. Usually, there is some lift on the front trigger while setting the rear trigger. But, you don't have enough pressure on the rear trigger to trip the lock. I would think about slightly taking just a little metal off the front trigger and possibly tightening the rear trigger spring to give it more pressure on the rear trigger which is what actually trips the lock. When I set these up I do the front trigger first without installing the rear trigger so it has a little bit of play, then install the rear trigger and spring and then adjust that trigger.
Nothing much to do on these sometimes but to fiddle with them. Some install with no problem, I actually had one recently that worked perfectly the first time installed with no adjustment needed whatsoever, others, of the exact same make can make you pull your hair out for days to get right.
