Hi William:
Prime Infrastructure is going to get this information from SNMP polling the devices. To confirm that the client's information is in that switch, check "show cdp neighbors" or "show arp". If this is a layer 2 switch, it won't know about the client's IP address, so there wouldn't be anything there to send when Prime Infrastructure comes asking.
Make sure that the device that is these clients' default gateway is also added into Prime Infrastructure, as that's where most of the IP address-to-MAC address mapping is going to happen.
In larger networks where there are more than 20 switches being managed, bug CSCue87691 keeps more than the first 20 from being polled, and that should be fixed soon.
Prime Infrastructure 1.3.0.20 Update 1 is out. Even though it may not clear this specific situation, there are a lot of good fixes there you'll want to have.