There's some way they can adjust the shift point of the car, maybe that has something to do with it, the bowden cable. Maybe that was tweaked a certain way previously for your old transmission and now it needs to be tweaked to work with your new transmission. But it's like when you hit a certain speed (or RPM) it will make the car shift.

Also there's a transmission vac switch or something that might need to be replaced.

Good luck.