Chronic breathing issue with brown leghorn hen

[TxJenHen]

This is the same hen that I have posted about in the past. She has a chronic breathing issue where her comb and wattles turn purple. I give her LA 300 and I vet RX her very well and she does OK for a few days and then it is right back. I only feed them layer crumbles and water with poultry vitamins. I keep scratch out of the co-op and I also give oyster shell crumbles because this same hen lays soft shell eggs. I don't know what the issue could be because the rest of the flock is never affected but that is this is a chronic problem with just this hen. Please help