What version of ArubaOS is this? (ArubaOS 6.3 and above by default uses ClientMatch even if bandsteering is enabed).
What access points are you using? (Some access points do not support band steering by default)
Clients can still move back to the 2.4ghz band if it sees it stronger. In the end, clients make the decision, they can be influenced by features by ClientMatch and band steering. If you see too many clients on 2.4ghz band, you should lower the 2.4ghz power relative to the 5ghz power to encourage the client to choose 5ghz more consistently.
"show ap association" will tell you how many clients are in each category.