I agree with Jack, definitely need some more information to properly understand the problem before a potential solution can be offered.
Assuming you're peening .012-.016A and not C scale I would say that 280 is probably too large. 170-230 55-62 Would be better. The larger the size of the shot for a given intensity, the longer it will take to get 100% coverage.
Without knowing this specifics of your machine or the part you're peening it's hard to tell, but I do think your feed rate is extremely high.
What are the 4 axis of movement?
Is the spindle the turntable or robotic arm?