Sounds like your bulbs need to be divided. Here is some good info for you from the American Daffodil Society:
My first question would be whether anything has grown up during that time to shade them. Have they bloomed well until this year?
I agree with Douglas. If they are in deep shade, I would move them.
YOu could wait until 1/3 of the foliage has turned yellow or dig up clumps now and transplant.
They are not thickly planted. In fact, they are scattered around only in light shade. Could they have become too deep over the years from leaves and mulch??
How much of a layer of leaves and mulch is over them, Karen? Have you noticed the decline over time?
they have declined over time....I had mulch about every two years. Most leaves blow away eventually.
If the mulch is affecting the daffodils, it is probably too deep for other plants as well. If you have never fertilized them, apply a complete garden fertilizer such as a 6-24-24 or other fertilizer with a 1:4:4 or 1:3:3 N-P-K ratio. Try to pull back the mulch before applying, which, in addition to increasing the effectiveness of the fertilizer, will give you a chance to assess how deep it is.
If they have never been divided in 15 years....this is definitely your problem....you dig them up in September for division....plant in nice sunny place for the most blooms....you can use bone meal when you do this....they love it....just like tulips. Oh rule of thumb for planting is 5 times the bulb
I had a clump that never bloomed. I moved them and planted not at the recommended 6" and they have done great ever since.
Here's a Tip on planting seeds. For some time I could never get my seeds to come up.I was actually washing them deeper down in the soil. Well a gardener Told me to buy wide Painter Tape ( blue ). Press your seed onto the tape just a few. This will keep your seeds from being washed away. Good luck