Client Computers dropping off SEPM

I have noticed that if a client PC has not been switched on for some time (possible a month) it drops off SEPM, and does not show up in any of the reports from SEPM.

Whenever the client PC is turned on, it will automatically be added back into SEPM.

Is there a way to prevent clients from dropping out of SEPM after a prolonged period of not being used.  I would like SEPM to be a complete record of all clients, reporting on absence machines so that we can address them.

Regards

Eamonn