About the deficiency, I would check the soil alkalinity. From a few angles, I also wondered if there was powdery mildew but could not decide for sure. So, I will mention it in order for you to look at the leaves more closely than I can and determine if you see the grayish spots common if PM. If you do not notice any then "never mind".

About the size, it may be a hydrangea macrophylla that blooms only once a year. The flower buds form in late summer or early fall. But then winter temperatures or a late frost kill the stems so it starts growing all stems from the crown every Spring. Since the previous year's stems get killed, there are no Spring flower buds and you end up with no flowers. The only way to help it will be to (1) make sure you are not fertilizing excessively (once a year in Spring is enough), (2) no pruning and (3) add winter protection when the plant goes dormant.