A couple of thoughts... If the drifts are large, healthy and full, it's possible they are receiving to much nitrogen. If you've fertilzed them with ordinary flower type fetrilzer, that could be the culprit. You also state that some of them have been there for a while,maybe quite a while. I'd recommend waiting until the leave yellow and begin to die back (as you've already mentioned that you do) and dig up enough to retrieve some of the older example. They should be divided at least every two to three years. This should help significantly. Are they in a sunny area? Daffys definitley like enough un. We had some that did well for a few years but began the non-flowering routine... a neighbor noticed some red currant bushes we had later planted,had matured and blocked some sun... moved the bulbs and they did well again.
A parting thought, if the bulbs are planted near your lawn where they might receive grass fertilizer... hmmm?