I had the same problem a few years ago. It could be that you were given small bulbs that were first season offsets so not big enough to flower? Mine all seem to flower fine now. At the end of April, water them with a broad spectrum liquid fertiliser just before they die down to help build them up. I think they will be fine in the long term.

The problem is that snowdrops, or any other bulbs, don't like being disturbed whilst they are going over. After flowering the bulbs take up energy from the soil to build up strength to flower the following year. If you dig them up you disturb the fiberous roots beneath the bulb and they stop growing for that year so, few flowers next year. This is why I don't like the practice of selling bulbs 'in the green'.
Leave them alone, or give them a feed of tomato fertiliser as Fractal suggests, and they will be OK within a year or two.