A sad fact is that although the tulip is classed as a perennial, it doesn't necessarily keep on blooming year after year. If you get 4 or 5 years worth of flowers, you will be VERY lucky, and many varieties will only give a good display for one season - or two at a stretch. How long have you had yours for? If it's longer than a few years, it might be best to dispose of them altogether, and replant in the autumn for next spring.