1. How old are the bulbs? Have they been in place long enough that the clumps have grown substantially and become crowded? If so, they might need to be divided.

2. How much sun do they get? They really need full sun in order to bloom well every year.

3. Any problems with the soil? Maybe all they do need is a good bulb fertilizer. Espoma Bulb-Tone is a good one.

Those are my best guesses--need to be divided OR aren't getting enough sun OR need to be fed (or any combination of those three). I am assuming you haven't seen any sign of insect damage or foliar disease.

Dawn

Dawn makes a good point. In my experience (I don't do that many Dafs) two things stand out - overcrowding and lack of prolonged winter chill. This year it seems like we had more cold days then in the past couple of years and my dafs have bloomed better then ever. If you are having problems it may be overcrowding. I can't advise as to nutrients as in my heavy clay soil we have excess amount of the Phosphorus (the nutrient that causes lots of blooms). If you have sandy soil an analysis from your ag extension service may be helpful. Also, if the foliage emerged properly but then you got no blooms it could be that the buds were killed by (A) a late freeze or (B) drought as they were forming. With dafs at least there's always next year.

Daffs need full sun during their growing season--Dec to June. After that they go dormant. I have hundreds of daffs under deciduous trees. But they must be divided when they get crowded. This is the most common reason for bloom decline. The best time is late May. I like to put them right back in the ground, but I have taken open boxes of dry bulbs in the house and kept them until Sept. I have also planted them by just tossing them on the ground in the woods next to our drive in early Oct and letting the falling leaves cover them. My early daffs, which normally bloom late Jan to early Feb, suffered quite a bit of bud kill this year. Too daffs don't like to be planted too deep and if for some reason the soil has filled in over the bulbs that might be a problem.