Intra-ESS roaming (i.e. roaming between access points with the same SSID) is typically performed at the WiFi driver/chipset firmware level. It is unlikely your Operating System is having any input into the decision making process.
Inter-ESS roaming (roaming between APs with different SSIDs) is done at the supplicant level, and this is when your OS/supplicant will have a say in who to connect to.
So you need to look at your WiFi card and driver, and check that you are using the most up to date one, and that there are no known issues.
Also, as pointed out by others, with signal strength, the lower the number the better. Chipsets I have worked on usually initiate intra-ESS roaming when the signal strength hits about -80dBm but this will vary between manufacturer.
You could always try walking around with your laptop and watching the signal strength as it drops to see when roaming is triggered.
Please do see http://superuser.com/q/844471/243625
– zagrimsan – 2015-09-18T14:58:33.873