Answer: Based on your description, the tree seems to be stressed. An extra heavy blooming (the catkins) can be an indicator of stress, although they don't last all that long under normal circumstances. Yellowing and sparse growth/foliage certainly indicate there is a problem which can be due to poor rooting, over or under fertilization, a pH that is too acid or too alkaline, overwatering or poorly drained soil, or a pest or disease problem.
It is very difficult to diagnose this type of problem long distance. I would suggest you work with your professional nurseryman to try to determine what is happening to the tree. Part of that process should include running some basic soil tests and a careful inspection of the trunk, branches and foliage for any unusual signs such as insects, holes, sap dripping out and so on. Also check the graft to make sure it has not cracked or split or been otherwise damaged. The staff may also appreciate an overall photo as well as closeups of the foliage and possibly a freshly cut foliage sample showing the yellowing, it should be kept cool and enclosed in a clear plastic bag. I'm sorry I can't be more specific for you and good luck with your tree.
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