I suppose that I am answering my own question somewhat. I found some more info that said that apple root cuttings should be taken in late winter before any signs of the tree coming out of dormancy. I took root cuttings in the spring after the tree bloomed, so maybe that is why only 1 out of 30 grew?

Any other info would be appreciated. Thanks.

Most trees root best with cuttings taken in spring just before sprouting, but there are exceptions. Like grapes, apples root better in winter. In fact, I have often had cuttings from winter pruning that fell to the ground and the cut end in the soil, missed when picking up after that rooted. They were 2-3' long. I have two bonsai apples 16 years old that started that way. In our wet climate, they are kept wet enough to make it through winter and sprout in spring, but you might have to keep them watered. Rooting hormone will increase your chances of success. If you are working on root stock for an orchard, I hope you are young!

You can make rooting liquid by soaking willow branches. Not sure if that helps but I think it's cool.