Windows only supports FT over the networks with 802.1X (i.e. when using WAPx EAP), it does not work in open networks or networks with WAPx PSK. That does not mean Windows laptops does not roam at all, it just meas Fast BSS Transition is not supported in those cases.
When using the new CAPsMAN, however, I used to struggle with a couple of Windows laptops that were stuck on one AP and refused to roam even when the signal was dropping way below acceptable level, no matter if FT was enabled or not. This has been fixed for me with the following setting (follow this link if you need some explanation):
set ... security.connect-priority=0/1