There are several reasons why an apple tree does not produce a full bloom:
It is possible that the tree is too young and is just beginning to become reproductive, that is produce flowers. If this is the case, flowering should increase next year.
A young tree may be overly vigorous, spending its energy producing wood and leaves and not flowers. Over fertilization is a possible cause.
Flower buds are formed 10 or 11 months prior to bloom, that is the previous spring and early summer. If the growing conditions in that previous season are such that the tree is weakened, it may not be able to set flower buds for the following season. Examples of such conditions include drought or excessive water and/or disease.
Many apple varieties want to be biennial producers, that is they produce a large crop every other year, with only a minor or no crop in the intervening years. When it blooms heavily and many of those flowers set fruit, the energy of the tree is focused on developing those fruit, and it does not expend energy to produce flower buds for the following season. So, in that following season, flowering is slight or nonexistent, but the tree will focus on producing flower buds for the following season. Hence, it develops a biennial cycle.