hello are
If you set it to prefer then they willl invite clients to that band... Try using prefer... dont force if it you can
There are many factors you need to take in mind.
1-Does the design was base on a 5ghz site survey?
If it was not.... or you really dont know... it could happen that the guy that did the site survey, did survey on 2.4ghz and you got small isle of 5ghz... why this happen? because 2.4ghz travel farther than the 5ghz signal..
So If the user is at the edge or barely can see the 5ghz signal then he migth try connecting to the 2.4ghz or if he cant see the 5ghz band... because its too far and he can see the 2.4ghz (got the idea here?)
2-Does the clents on their setting got to by default connect to 2.4ghz? does it has any preference? check that out.. i have seem some wirleess card that are abgn and for some reason... i dont know it has it use 2.4ghz liek forcing it to that band...
Start checking those 2 things....
If you got something like an airchecker ti would be easy for you to figure out the 1 thing i said.