Another few possibilities for this which were not mentioned in the article:

* Check that all your APs are broadcasting the same name. No devices I know of will naturally roam to an AP with a different name.

* Check that all your APs are broadcasting identical security features (e.g. WPA1, WPA2, WPA1 or WPA2 .. as well as the encryption methods like TKIP, AES/CCMP, etc). Many devices will check these too, and roam only to matching APs.

Some devices also have a AP priority list, it's worth checking out if they can be modified (e.g. like on macOS).

Yeah, same name and security (otherwise it would presumably never work.) The problem is the in-between area where there's bad reception for one and good reception for another.

There are protocols for roaming, like 802.11k, 11r and 11v.