Frost during pollination could be a culprit, but more than likely the lack of flowering and fruiting could be cultural as well as transplant shock. It takes at least 3 years or more for a tree to adapt to its new location. We see the same thing in lilacs that flower the year after you plant them, then the next year, you don't have flowers as the plant is expending its energy into producing new roots at the cost of producing no flowers or fruit. Flowering and fruiting are energy draining processes that can come at a cost to new root development. If your tree still does not flower after another year, there may be other problems. Mountainash are very susceptible to fireblight, which kills stems and can eventually kill the entire plant. Depending on the species of mountainash you have, it may not be flower bud hardy to your area. The native and European mountainashes are hardy up north, but the Korean and some hybrid mountainashes are not. If you had a severe winter, the flower buds might have been killed whereas the vegetative buds are more cold hardy.
