Analysis of the Reasons Why Young Chickens Keep Developing Paralysis

Recently I have often encountered 50-100-day-old pink-shell laying hens continuously developing paralysis at a rate of about 1%. Some technicians who looked at them said it was Marek's disease, but after dissection, there was no swelling of the sciatic nerve and no tumors. Moreover, even in the same batch, the brown-shell laying hens did not have it, and only the pink-shell chickens did. What is the reason for this? I hope the experts can shed some light.