You are doing everything correct. I've grown a couple of weeping cherries and had similar experiences. I pruned just once a year after blooming, removing dead branches and nipping back the longest ones that had begun to sweep the ground. It's pretty common for transplanted trees, especially the ones with a caliper greater than two inches, to spend several years establishing their root systems before growing in trunk girth. One of my weeping cherries in particular grew a terrific and vigorous canopy right after transplant but it was three or four years before the trunk caliper thickened. If you haven't already mulched the tree, add a 3-inch-thick layer of organic mulch like shredded cedar or cypress. The ideal mulch ring extends out to the dripline of the branches. Mulch will aid the root system and prevent bruising of the trunk if the lawn mower gets too close.