hi
L2 roaming default settings enable if you want L3 roaming and if you want wpa key fast L2 roaming you need mobility lisance
There are a few reasons for this problem to occur
1-some client don't support roaming
please test
connect win7 client you wifi system and ping another destination for example default gateway
roam all sites
check any connection issue if you want many ping loss your wifi system wrong design.
if you not see any loss packet to be wrong phone system design
2-if you use encryption wpa this fault very normal
change or disable encryption and retest
3-check and/or make wireless site survey report
with Hp RF planer or another program
may be your access point wrong place
http://h17007.www1.hp.com/us/en/products/network-management/HP_RF_Planner/index.aspx
good luck