Why Can't Caladin Heal His Brands?

Lopin is able to re grow his arm, but caladin cannot heal the brands on his forehead. Lopin gets to be squire wind runner because of caladan. In lopin's spirit web, he has a full arm. And so when he gets this power, he regrows his arm because part of his spirit web. But calidan has changed his spirit web because of the torture and abuse in the physicality thatver ou say to say, "I am dangerous ritly"