Hello, and thanks for your question and the good photos. From what I can see, the majority of the foliage looks healthy and this is a good sign. A few underdeveloped leaves turning yellow is not something I would be too concerned about unless it starts to spread throughout the tree. Most likely there is an environmental cause for those leaves to stop developing including availability of water, cold temperatures, nutrient availability, or a combination of these factors. I don't see environmental or plant disease symptoms on any of the fully developed foliage in the photos.

Continue to provide general care for the apple tree; regular watering and fertilizing, annual pruning, and pest management as needed and this apple tree will continue to grow and thrive.

Here is some additional information on caring for apple trees: http://www.ext.colostate.edu/ptlk/1201.html

Please feel free to contact me if you have other questions about your apples.