Unfortunately, band steering is an inexact science. The AP can try and detect whether the client is dualband and offer the 5Ghz band, however it is ultimately up to the client to decide which it wants to connect to. The exact software implementation varies so much by vendor, that it really depends on the client device, what it's running, who it's made by. The key issue is that the 802.11 family of standards specifies that AP selection and roaming is driven by the client, unlike in cellular networks where it is controlled by the network itself.
Sometimes clients also insist on locking on to the 2.4Ghz band because they detect a stronger signal and thus prefer it - 2.4Ghz propagates better than 5Ghz. You may want to carefully lower the 2.4Ghz power, bearing in mind the other effects on your wireless coverage area.
