A scab could be caused by many things. The fact that it bleeds sometimes could be due to other chickens picking at it or that chook scratching at it.
If this is slow to heal, healing might be prevented by others pecking. The longer it takes even a small wound to heal the higher the chance of infection. You might consider isolating this one, applying an antibiotic salve daily, and seeing if it will heal up.
You didn't say how long this has been going on. Hopefully this chook is in good health otherwise. No matter if you choose to isolate the chicken, I would suggest some salve to possibly help with healing.
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