Please read J443 and J442 its a really easy to read spec for your questions about strips.
To answer the topic, do as Jack suggest. Increase the "dent makers".
I always start trying the highest possible shotflow. Some times you get a saturated strip with just one pass. Then you need to increase distance to part and maybe use a nozzle with wider spray.
I also suggest to look into your dwg requirements. If its old it might tell you to use 200% coverage. That is a major waste of time and is not good for the part beeing peened. Tell construction department to change to 100%.