It could have something to do with temperature. I know that sometimes nectar from some flowers (abutilons, fuchsias etc) is sweet and attractive to hummingbirds and other times it is just watery and plain. This was from my own taste testing. Maybe taste your dandelions!
yes, there is somethind odd in dandelion nectar. I have seen that when weathers are really good, flowers do not give nectar. I think that they grow so fast in moist and warm weather the sugar content is too low.
Often hives get 15 kg honey per hive, but if next week is rainy, hives eate all.
Sometimes I have got 40 kg per hive.
However, dandelion is important here to all hives' build up.