Lee, there will always be adamant clients which won't get steered to 5ghz even though there is 5ghz WLAN available as preference is set to 2.4 ghz on those clients however in some clients i have noticed that you can tweak this behaviour by making 5ghz as default.
Or set the 5G band as a different SSID.
or if this is specific AP area issue then you can just turn off WLAN service for that ZD controlled AP for 2.4 ghz so that client will be auto forced to connect to 5ghz.
Hope this helps.