If it doesn't cover to voicemail at all, check coverage group members, to be sure it is assigned. Extensions>More>Group Coverage>30>Enter>Inspect to see what is already assigned, Extensions>More>Group Coverage>30>Enter>Ext. No.>Enter to assign
If you go from the automated attendant directly to the mailbox, without ringing the phone frst, see Dexman's advice above.
If the phone rings, but the mailbox is not recognized when it covers to voicemail, confirm that the mailbox has been created.