Baby signing in a totally different context

My almost 12 month old has started to sign "more" after I finish reading a book. He will sit and listen to me read the book a second time and then sign more again and again and just let me read the book over and over.

I have only done the "more" sign when feeding him and he doesn't use it when eating at all. Even when more of the food he wants is out of reach, he will point and grunt to the food instead of signing, even when we demonstrate and wait.

Has this happened to anyone else? How can I get him to do it in an eating context? I was starting to worry that he wasn't signing at almost a year old. Admittedly, "more" is the only sign I do consistently.