Elm tree buds have an inherent problem.  The buds don’t always completely grow after they start to swell in late winter. They all start to grow.  But they don’t all continue to grow.  This became apparent in my work with a majestic Elm Tree today.
Problems with buds means less active growth.  Less active growth means the tree is more susceptible to diseases and insects.  This could contribute to why Elms were so devastated by the Elm Bark Beetle and Dutch Elm disease.