Dave's garden says the snowflake clematis is :
Group 1 - Spring bloomers; no pruning
Grow in fertile, medium moisture, well-drained soil in full sun to part shade. Roots should be kept cool, shaded and uniformly moist. Bloom occurs entirely on the previous year's growth (old wood). No pruning is necessary, but, if desired, should be done immediately after flowering. Needs adequate supply of nutrients during the growing season to support rush of growth. This species is not reliably winter hardy in the St. Louis area where it should be planted in a protected location.
You didn't say where you are. If you are in a cold winter area, that may be setting it back so that it doesn't bloom as well. I'm in zone 6 (which I think is similar to St Louis) and I have had a hard time growing clematis (hot humid fungal summers conducive to clematis wilt as well as cold winters). It should be hardy in my area. But just in case I brought the one I planted last year in for the winter, after it had gone dormant. It is now out there growing by leaps and bounds and flowering. But then so is the one in the front yard that stayed out, nearly died a couple years ago, struggled along last year without flowering and now is flowering like mad. Maybe it likes the cool wet spring we had?