You can either alter the default post session timeout value from 30 minutes, to a value that makes sense for the client. If it takes 20 minutes for the client to roam between these APs, perhaps set the post session timeout value to 15 minutes. Another way to resolve this would be to force the client to re-auth upon roaming between APs or areas. click VNS Tab > WLAN services section (Selecting WLAN service you are using) > Auth & Acct tab > Configuration button next to "Enable MAC-Based Authentication" as seen here:
Drop-down the list next to MAC-Based Authorization on roam" and select option either on inter-Area roam, or inter-AP roam, Click OK, and then click save.