MCPE Game not full but says it is

My grandson downloaded a world called Cobblestone Clicker, and my granddaughter and I both want to play. We have a (possibly unusual?) problem:

I can join the world on my Kindle Fire gen 7; he can join on his Kindle Fire kids' 2015 device, and I can join on my Android Pie phone. However, nobody can join on my DGD's Kindle Fire device, nor can anyone join using DD's Blackberry (I know), or on her Kindle Fire generation 9.

We are on the exact same wifi. We have closed down MCPE, shut off devices completely, restarted the router ... repeatedly.

Can anyone shed light on this issue, i.e., what is the way to allow the other devices to join this world? The message always says, "Wow, this world is popular! Check back later to see if a spot has opened up." This happens when DGS is the only one shown in the world. TIA for any help!

Check the number next to the world. Do the numbers match up? If not, tell DGS to type this in chat:

/setmaxplayers 30

This sets the players to the maximum amount so you can join. Hope this helped!