Difficult to tell you much because we don't know much regarding your equipment, ball speed, the lane conditions you bowl on and so forth. But most of the time leaving a 7 pin consistently means that you are reaching the breakpoint a foot or two late, and/or throwing the ball a little to hard to get the 4 pin to kick off the 7 pin rather than around. I find that moving my feet a couple of inches forward can conquer the 7 pin (providing you play the same line). I'm sure there will be others providing their ideas- it's something that I faced in the first game I bowled tonight (5 strikes, 6 7 pins), and I made a slight ball speed adjustment (lowering 1-2 mph) and then I left only 2 7 pins the rest of the night...

Matt