Ah that's a shame, they do establish better when planted out in the green, but if dry bulbs are all you can get go for it. They are readily available here but perhaps more popular? Once established they will possibly seed around themselves. Some of the snowdrop displays over here have taken generations to establish. The National Trust garden at Wallington Hall which is nearby has planted 400,000 snowdrop bulbs since 2015! My sister has lots of different named varieties as well as the common nivalis and from single bulbs she now has several well established clumps.
