Can you tell us the following:
1. Are all the IAPs in the same vlan or different vlans?
2. What version of code are you using? v. 3.3.0.1 is the latest.
3. What are these devices?
4. Have you update the drivers on wireless clients?
-If the IAPs are in different vlans then you need to configure L3 Roaming on the IAP.
-Roaming is a function of the client. Often, poor roaming behavior is the result of old/bad drivers and often can be resolved by updating the client.
-Roaming behavior on some clients can be modified in the driver settings. For example, the Intel clients have a setting called "Roaming Agressiveness" which can cause the client to roam more easily or be more "sticky" and stick to one AP.