I was at the same level for 3 three years. I changed my approach, delivery, number of steps in my approach (this is all the result of lessons). My average didn't go up, but it didn't go down either. I consider that a minor victory.
This year, however, my average is up almost 20 pins. The difference? This year my goal was to pick up one more spare per game, on average. Sure enough, this year I'm picking up my spares a lot better than I ever have; and the only thing I changed was my intensity level. Now, no matter what I'm shooting, (baby split, single pins, 5 pin, corner pins, 8, 9, etc., it doesn't matter) I try to focus and execute my spare shot with the same intensity that I use to execute my strike shot. The results speak for themselves. I used to have a couple opens per game, now I only have a couple opens per set. I'm closing in on the Most Improved Bowler in my league. And the only thing I changed was my focus and intensity level by making every spare shot a strike shot. It may not work for everyone, but it sure worked for me.
Good luck.