Go to failure in every set but choose the weight that will to do this and get your rep range. So start the 8 rep cycle with the weight you are comfortable with. Do both sets with that weight for 8 reps and in a set you will increase the weight by 5 pounds for the same set next week. Otherwise keep the weight the same and try to rep out next week. This lets your body tell you when and how fast you need to progess.
And definitely like said asooneyeonig said lack of strength in one muscle group because usually when you hit failure in the bench press it's due to one or more weak area. So gradually strengthen the weak spot and you'll see your bench go up in no time!