my beekeeping mentor (who has alas very little English, we live in southern Germany) has asked me to tell you of his problem and ask about your experiences/ideas:
It is now about the 10th swarm in short order, where the queen did not return from the mating flight. He’s a bit at his wit’s end, so has anybody of you made similar experiences, has any idea about possible reasons and remedies?
He works on classiclal hives in German National.
If you need further information, I will relay to him and come back as soon as I have them.
After reading this thread, I’m wondering how he would go if he took his colonies that are making new queens to a different location. The first thing that comes to mind when queens don’t return is bee eating birds. Maybe the birds are hanging around the drone congregation area for an easy meal.