It sounds like they lacked dominance on the tree - the tree felt there were other growths it would rather feed to put leaves at that height. You say they are high on the tree but if there is enough competing growth higher they may not sprout. Sometimes a graft will look like its in a dominant enough position but won't sprout anyway. I don't recall that happening on apples for me but on peaches and especially persimmons it can be a problem. I have gotten these stuck guys to sprout sometimes by sacrificing more of the main tree. They may also sprout next spring.

Scott

Could notching be used to encourage growth?

Notching below the bud would be my suggestion as well.

Scape

To produce growth in a bud or branch, you would notch above that bud or branch.

Notching below a bud or spur is sometimes used to activate a fruiting bud or fruiting spur.