The only time I've ever had that happen is when there is a new queen that the hive is waiting to start laying or if there are laying workers.
Since you've been giving them frames of eggs there is larvae to prevent the laying workers from kicking into gear so we can cross that one off the list.
When you swap a frame for a new one with eggs, are you examining the other frames for clues? If so, what are you seeing? How long have they been without a queen? If it's been less that 30 days since you last saw eggs, then there may be a queen. That window is long by most people's standards but has worked for me.
There are probably other reasons that I don't know about, that's just my experience.