Additionally, should it associate to aa:aa, and then for some reason aa:aa goes down and bb:bb is the next closest AP, if the Verifone device won't probe for a new BSSID, Clientmatch will have nothing to steer anyway. The client has to ask for something and if it's not, it's a severe device issue that Verifone should look at.
We had similar issues YEARS ago with some handhelds that when they powered up would build a BSSID list of available APs, but once roamed outside of that and didn't see them anymore, would fail to associate. The kicker was with a wireless sniff, we would see the probes going out and the responses being sent back, but the handheld would just ignore all the probe responses. Vendor provided a fix (that led to another bug of a limit to the roam table), but that was a fun one.