Hi Friend,

The switch will not learn a mac address till the time device connected to it send some request out from its source mac address.

Like a workstation if you connect to it and your pc is in hut state the switch will not learn a mac address.

Incase you power your pc up it will sned a broadcats ofr arp entry and that time switch will learn a mac address because now pc has send some request with its mac address.

So just check which all devices connected to the switch for which it does not learn mac address are those devices active and up in the network.

HTH, if yes please rate the post.

Ankur