I don't know what the answer is but I have found the following on another website which you might or might not recognise
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Your foot slapping problem is similar to mine, which also resulted in metatarsalgia in my left foot. I finally figured out that I was concentrating too much on running B-H-B (Ball/Heel/Ball) and I had not yet learned to keep my feet beneath my center of gravity, so what I was doing was reaching out in front with my legs, then trying to force my feet to land B-H-B. This is not how feet were meant to be used!
I now realize that if I keep my knees slightly bent, keep my feet beneath my center of gravity, and run "from my belly" that my feet naturally land with a B-H-B movement and I don't even have to think about it! Plus I don't have to slam my feet down to meet the ground - I just wait for the ground to rise up to meet me!
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Sorry not to be more help.