Yellw Bellys Flakey Shell

(Plymouth, UK)

I have two young yellow bellys and ones shell looks flakey and thinning around the neck and just at the first tip of his shell. I have added a picture to try and show the areas effected there are two areas just off the centre and the very light part right in the middle of the shell. is it normal?, a lack of vitiums, to much light, does anyone know the answer?