When the subject behavior occurs, the Network Connections window does NOT indicate that the access point has been blocked.
This problem will occur if the computer is unable to connect during the initial connection (handshake) phase. When this occurs, the preferred access point wireless network connection is internally marked as blocked, causing it to be ignored when you move from one access point to another.
To workaround this problem:
1. In Control Panel, double-click Network Connections.
2. Right-click the network connection for the preferred wireless access point and press Disable.
3. Right-click the network connection for the preferred wireless access point and press Enable.
4. Close the Network Connections window.