Hmm, interesting. Perhaps an issue with your wifi design? Or are these sticky clients?
Client devices choose when and how to roam based on their inherent algorithms - it's not the APs job to handle that.
Generally it's only "sticky client" devices like laptops, HTPCs and some IoT devices that cling to weaker signals. Simply power cycling the client devices or toggling their wifi on and off (if an option) is successful at triggering a re-scan and association to the best BSSID.