it's often up to the clients to make the choice, and sometimes they just don't want to move.
Make sure you dont have the "min tx power" set too high - it can cause stickyness as you describe. You can also consider to raise up the minimum tx rates (wlan -> advanced) which has the side effect of reducing the coverage around the AP.
is your barcode scanner 2.4ghz only ?
what's the setting of tx power min/max ?
what are the APs typically using for tx power ? are they approximately the same or varied ?
is it sticking on the AP275 or an AP215 ?