I have been having the same problems. I found this website about the inter-access handover feature which states:

Inter-access point handover allows Wi-Fi® enabled BlackBerry smartphone users to roam from one wireless access point to another within the same Wi-Fi network without losing the Wi-Fi connection. The wireless access points must share a common service set identifier (SSID).

When creating a Wi-Fi profile on a BlackBerry smartphone, inter-access point handover is disabled by default. To enable inter-access point handover for a Wi-Fi profile on a BlackBerry smartphone, complete the following steps:

Click Options.
Click Wi-Fi Connections.
From the Wi-Fi Profile menu, click Edit.
Select the Allow inter-access point handover check box.


From the Edit Wi-Fi Profile menu, click Save to enable inter-access point handover.