Clients only connecting 11a/11g

[akcampbell]

We currently have two 9800 WLCs serving our schools. Their configs are virtually identical. When clients connect to one specific SSID on 9800 A, they'll connect at expected speeds using 11ac or 11n protocols. However, when clients connect to the same SSID on 9800 B, they only connect using 11a or 11g protocols. RF settings appear to be the same across both controllers. Any ideas?

[Rich R]

> As far as I know we're all good on the IOS side and respective patches and service packs.
> IOS version 17.9.4.27
If you had APSP8 installed then the AP version would be 17.9.4.208. 
17.9.4.27 means 17.9.4 or 17.9.4a with no APSP installed.

> I've not gone through each line between the two, just compared settings between the two in the GUIs.
Do the line by line - there are plenty of tools which make it really easy to diff the configs side by side (even Notepad++ can do it).  It's too easy to miss things on GUI and there are some things which just don't show up in GUI.
Also do "show ap tag summ" to make sure all APs are tagged as you expect them to be.