I have had limited sucess, 4 out of 10, when grafting to pencil thick seedlings. Is it my technique, or should I be using another method
when using rootstock seedlings. Most are pencil thick some are as thick as a thumb. I was thinking maybe the lack of a canopy could cause some failure. I noted on your tutorial, you were using an adult tree with canopy. Would appreciate your advice. Bill

Seedlings are best budded instead of bark grafted.

Bark grafting or budding, it is also great to keep them in shade while the healing process is going on for about two weeks in spring.

Are you using toothpick sized budwood when bark grafting to pencil size stock?

I have had good success doing that, but I am not doing it on seedlings, I have been doing it mostly on rootstock shoots from existing trees, but it has worked really well for me (once I found out clementines don't like satsumas).