I've had this problem quite often although I'm not using load-balancing but just band-steering.
I'm not sure what type of clients you have, but we use mostly Lenovo T400, T410, T420,and T430's with different Intel adapters and removing the full Intel Wifi Software Suite and replacing with the latest driver-only package + changing the adapter settings for Roaming Aggressiveness from "Medium" to "Medium-High" fixed 95% of those issues. Before we would have notebooks that refused to roam when going to conference rooms, etc.
I know this probably isn't the answer you want to hear (far easier to control it on the AP side) but it was the fix for our environment.