2 suggestions...
1- increase your barbell bench and overhead presses. Those you can increase at a more consistant rate and the gains there will probably transfer to this assistance exercise.
2- increase the volume of a lower weight before putting 40 in your hands. check your ego, and get to where you can do sets of 10 w/ 35 first. WHen you can do multiple sets of 10 of 35lbs, then move on to 40. It sounds like 40lbs is too close to your maximum to use it to make gains.
3- form. roll your shoulders back, make sure u have yourself stabilized with your chest arched to give you a better mechanical advantage
4- triceps. incline DB press is heavy on tricips, for me anyway. Do more Tri assistance work like dips and lying triceps extensions.