Avoid a location where there is a lot of traffic back and forth past the cage. Choose fruits and veggies high in Vitamin A for their immune system benefits. A plucking bird is usually a stressed bird and may need the immune system boost that high Vitamin A foods provide. Often seeds are implicated in plucking. Some birds can have an allergic-type reaction to some components of seeds or the seeds may be contaminated with mycotoxins.
Peanuts are often a culprit.
They may be contaminated with mycotoxins and other bacterial or fungal organisms. Remove all kernel peanuts from the diet. Remove all nuts in the shell from the diet. Mycotoxins again are of concern. I don't recommend that birds be fed kernel peanuts or dry seeds or nuts in the shell.