Go to logs Page and try to pull the "Computer status" log for the "Past 24 hours" and see if you have clients showing up in the results.
Also check the followings
check if the number of online/offline clients are shown correctly in the home Page of SEPM.
Go to Clients Page and then highlight a group in whcih you know that you have clients and then go to "Details" tab of that group and check if the "Number of physical computers" is showing the same number as in the other (working) SEPM. If this shows the correct number, Open the display filter and set a smaller number (like 200) and check if the clients are then displayed.