indenting

Why is it so difficult for children to remember to indent each paragraph? We have been working on this all year long. They're publishing, and they keep forgetting, even after I remind them right before they are about to do it. Then they want to put the editing mark, and I asked them to show me in a published book where one was. Of course they couldn't. Sometimes they do it, but it's so haphazard. I guess one day it'll get stuck in their head that way. Or at least I hope it does. Their writing test was pretty good. Two weeks left before we go off track. There's a lot to do before that.