There's a setting on the AP-4000 (which was introduced with the 2.4.0 firmware) which controls whether the AP keeps records of connected users. On the AP, the setting is called "Enable Monitoring Station Statistics"; AMP calls it "Wireless Station Monitoring". On this AP, the setting is off, and when it's off AMP can't get user data from it. The only way to resolve the config mismatch is to enable the setting.
There are a couple of ways you could enable it:
1. Go to the Monitor/Station Statistics page on the AP's web UI and enable it.
2. Have AMP manage the AP. When AMP manages Proxim APs, it automatically turns on Wireless Station Monitoring.