Probably still putting roots down and concentrating on establishing. Be patient, it will probably flower better next year. Trees in their first year of planting shouldn't be allowed to fruit anyway, so you were lucky to get one plum! My damson had no flowers the first year, and two damsons the second. This year it is full of flowers, so I'm hoping!

Also double check to make certain that the non-blooming branch isn't coming from below the graft. That may be a non-fruiting rootstock variety, or it may be a seedling, which may, or may not, be worth keeping. Either will compete with the desired plum, and reduce vigor and fruiting.