Sounds like they have bulked up to many offsets, or bulblets. They will cease to bloom when all this reproducing has resulted in a major mass of small bulbs tightly packed together. Dig up the big mass, divide the bulbs, replant immediately. They will take 2-3 years for the bulblets to mature and bloom.
Other then that: loss of bloom due to lack of sun. Deciduous shade is fine, not evergreen shade. The other is the greens are being cut before the bulbs go dormant.

Paula is right. After 10 years they are needing dug and divided.

At that time I moved 100's of daffodil bulbs.
Don't be surprised if by now you have thousands of small bulbs. Be prepared to share them if there happen to be more than you can use.
If there are a lot more than you can replant at this time, you can dry, and store some of them for fall planting too.

The link below lists other reasons why daffodils won't bloom, but I'm sure crowding is at least one of your issues.

Sue

Here is a link that might be useful: American Daffodil Society-Daffodils Not Blooming?

Sue - I love the last sentence#13 in the link you provided. I needed that good morning chuckle to start the day. Thanks. Candy

#13. Bulbs may be out to get you! (The case when you give them away in frustration and they bloom wildly for the new recipients.

Ha! I've never had #13 be the case. Usually my bulbs that don't bloom, have needed divided for so long that they have just divided and divided and grown to be so small that once they are dug and divided, they might need several years to get big enough to bloom again.

I'm thinking after the foliage does down and it is the right digging time, I'll dig several non blooming clumps and just offer them up here at GW on the Plant Exchange for postage. I imagine it will take them a few years to bloom.