What causes yellowing of the leaves on camelias?

Try a feed of sequestrene. Mine is in a pot in ericaceous compost, but it still needs this about twice a season.

if its just an odd few now and again it is the natural shedding of old leaves. as they are evergreen they have to lose old worn out leaves from time to time. but as spritz says a good iron tonic will do it no harm.
i feed mine sequestrene twice a year and i am a chalky soil. mind it isnt a dark green leaf.

just like human hair.we all lose hair but balding men like me dont replace it as quick as we lose it