If you change the policy serial number in SEPM (by changing any policy), is the new policy serial number getting updated in the SEP client?
If the policy serial number updates, make sure that the client-server communication mode is set to "PULL" mode.
If the policy serial number doesn't update, then check if you have more than one domain in the SEPM console (as in the picture below). If there are more than 1 domain, switch to other domain and check if the client is shown as online in the other domain.