YES I THINK I HAVE ANSWER.
i Have had same problem for 2 months since ios8.
SOLVED. Because I use Apple's Air Port as Router I can actually see what each device is doing. (Just discovered this). You are right. Initially all my devices MacBook Pro, ipad mini, 5S , 6Plus, flip flopped between 5 ghz and 2.4 ghz One by one had to change to 5ghz and forget any others. Then saw
on the Air Port App in Utililies how one by one the Air Port Router reset. Then instantly all my files synced, unfroze and became active.
Then i went to each device to check on the same Air Port App , that each device stayed on the 5ghz Channel. The wifi speed was doubled if not trippled. (Compared to the 2.4).
So, you have to get each device to stay on 5ghz and then to get the Router to set/reset.
Tip. Once you set the 5ghz. Reboot device. Check and double check if device stays on 5 ghz.
I am not sure how can see all this inside a Router. The Apple Air Port App lets you see all this On Mac. As does the Uutility/Air Port App on each device.
I am convinced this is The solution. I tried everything before this as everyone else has, the entire 2 month saga. Apple was of no help!
Figured this out myself after over 2 months!