I'm not sure where you live and that makes a difference because Tulips need to overwinter in freezing weather. Here in Texas, we have to pull them up and put them in the refrigerator. That was too much trouble for me (you also have to be careful not to store them next to certain fruits and vegetables), so I only planted them once. On all bulbs, it helps to strengthen them by cutting the dead flower heads off after they bloom, but letting the leaves naturally die out (that is how the plant gets energy in the bulb for the next year). You can also fertilize them once. Another possibility is that they did get some fungus or other microbe this year, which may mean they are not going to grow again. Pull a few up and see if they look rotted.