I should also say that incline DB bench is probably being done when you are done closer to the end of your chest work...so you have probably already done your barbell bench and incline barbell bench. If so, you are already fatigued and now using this assistance exercise to just spur some muscle growth. Maybe you need more rest between sets.
You should probably also be rotating this exercise through with other assistance work every few weeks. Incline db bench gives variety and helps you in that when you rotate it, it gives your body a different stress to react to...but its not a main lift or one you should be beating yourself up over. Your time would be better spend doing drop sets of bench or OHP, or dips. Or just resting and eating. Focus on the regular bench...no one will every say 'Hey buddy, how much ya' incline db bench?'