To me it is a mystery why one plant blooms and the other four don’t.  There are five plants in that primrose bed and only one has flowers.  From experience I know they may bloom twice a year early spring and late fall.  I really don’t know why only one is blooming now.  It may be the light is just right.  I am only guessing.  I keep checking those plants.  At some point it will be too cold even for them.  The leaves will die and it will come up next spring.  Then I can enjoy them, again.