Active AP Rebalance Threshold=20% means a minumum total load % required to do AP LB.
On a 7205 that support 256AP this means that 20% is hit at 52 accesspoints at one controller.
Active AP Unbalance Threshold 5% means a difference between most and least loaded MC
ClientMatch balances the load periodically, when total load exceeds the minimum load threshold, and the difference between the most and least loaded controller is greater than the Unbalance Threshold.
With 40 RAPs the first criteria doesn't meet the Active AP Rebalance Threshold of 20%, both criteria must be met.
You can lower the active AP Rebalance Threshold to a lower number in your case.