Your larger Honeycrisp probably needs some pruning. It needs to be opened up to allow sunlight and breeze to go through much in the same way it now does with the younger tree. However, these trees cannot pollinate each other. There must be another apple tree (even an ornamental crabapple) in the area which will pollinate the blossoms. It could be that the younger tree is closer to that tree and insects just don't make it to the larger tree with pollen. It is too late in the year to prune your tree now but it should be done in March or so. Here is a link: http://www.extension.umn.edu/garden/yard-garden/fruit/apples-in-home-garden/  In the meantime, check out where the pollen is coming from for your trees.  Maybe you would like to plant a ornamental crabapple in the front yard to provide more pollen for the larger tree.  Also, the fertilizer was not the problem. Either fertilize your trees or the grass in the area. Also be aware that apple trees normally produce a lot of fruit one year and very few the next and so on.