I lead a group of what was a "no drop" ride and we had the same problem. It was a small club so there weren't enough riders to break into 4 groups. People who showed up wanted to ride anywhere from 12mph to 20mph even though the ride was posted at 14-17mph. How the rides worked was everyone stayed with the leader or tried to. Faster riders were wanting to go off the front because the group wasn't riding fast enough for them . The slow riders were struggling to keep up. So the ride would re-group too much, nobody was happy.
So we changed to cue sheet rides, the rides are posted at 15-17mph. If you want to ride faster or slower fine. We'll see how this works in the next few weeks.
IMO rides should be only for a maximum variation of 2 to 3 mph, anything wider won't work well.