Have you enabled the fast roaming on WM controller?
The bridging mode should be tunnel for the wireless lan.
If above setting are correct then try configure the wireless clients NIC card setting,
The following is copied from
Intel wireless client online help page:
Roaming Aggressiveness
This setting allows you to define how aggressively your Wi-Fi client roams to improve connection to an access point. Click
Use default value to balance between not roaming and performance.
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Lowest: Your wireless client will not roam. Only significant link quality degradation causes it to roam to another access point.
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Medium-Low/Medium-High: Allow Roaming.
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Medium: Balanced setting between not roaming and performance.
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Highest: Your Wi-Fi client continuously tracks the link quality. If any degradation occurs, it tries to find and roam to a better access point.