Probably one of two things, or maybe a combination of the two. First is possibly too much nitrogen from lawn fertilizer (if you fertilize lawn), or they are over crowded.
If you think it might be nitrogen, cut back fertilizing near them, and add some bulb fertilizer (high phosphorus) around the flowers when you are expecting some rains to help wash the fertilizer into the soil. Best timing is early when the daffodils just start emerging, but won't hurt now, and will be good for them if you think it might be part of the problem.
My best guess is they are over crowded, especially if they have never been divided. Wait until foliage turns yellow and the leaves start to droop, then dig them up and divide them. Put some bulb fertilizer in each hole, a couple inches deeper than the bulb. Don't put the fertilizer in contact with the bulbs. Plant bulbs individually, and give them room to grow. Somewhere around a 12" inch spacing is good, any closer and you may have to divide them again in a few years. Around 6" depth. May not see a lot of blooms next year, but if you fertilize now and divide later this summer, might just be surprised next spring.