Sterile Snowball Bush March 31, 2019

When I planted this large shrub / small tree in 2014, I was extremely impressed with its performance.  It has a very long bloom time and the flowers are large and pretty.  As opposed to the common Snowball Viburnum, this variety is sterile.  No seed forms, so it can expend all its energy on forming flowers.  But, the last time it bloomed was 2016.  And this year only 10 or so flowers formed.  I was actually considering removing it, but then I saw some buds on some of the branches, so it got a reprieve.  In theory I have tons of space to plant things, but my irrigation only extends so far  (and my human strength to build beds and keep them weed-free), so I want all-stars in my garden.  I don't know why it has not been performing - inconsistent watering?  voles? something else?