fifibreen. you have very nice set up, I like the big chicken, it's great! I too have sand on my run and coops and since I have had bumble foot problems in the three years I have been keeping chickens, I inspect their feet at least once a month and I have found small pebbles in the webs between their toes, I usually take them off and they are fine but I wonder if I don't check them regularly, the pebbles will stay there for a long time creating a similar problem like yours. I know this might not help you understand what is going on with your barred rocks, it might be genetic or it might be just happenstance but I guess my point is, we should take a look at their feet often enough and you are doing a good job clearing their feet regularly.