I think shaneystar has a good idea, but i would ask if you cut them back last year before the leaves turned yellow? This will certainly affect the following years bloom. Additionally, have they been in the same bed for a number of years, say 4 or 5? If so, the bulbs may need separating and replanting. I do notice though, here in the western U.S. at least, only about half or a little better, of the tulips produce blooms each year. I suspect it has something to do with how harsh the winter was, but that's only a guess...