Wilian,
There was a thread here in the boa forum some time ago about a boa with a similar issue. Do a search, (although I don't know what keywords will be effective), you may just have to browse through the forum.
But I believe it was about a young lady who had a boa that was essentially losing pigment with every shed. If you resurrect that post, you may find some answers.
You might also try a search on the forums at
www.boaphile.com
I seem to remember a similar thread there some time ago as well.
Good luck.