I swapped some servers from SNMP to WMI today and ran into the same issue - a discovery picks up all interfaces/volumes but CPU/Memory is not being polled unless I manually go through each server and List Resources, etc.
I tried to use the Manage Pollers but the Scan Results show Not a Match for all of the servers. If I manually go and List Resources for a server, select CPU/Memory, then go back to Manage Pollers it will show as Exact match and enabled.