symantec some clients are not updating

Please help me to solve this issue. Im new to Symentec. (Im using SEPM version 12.1.2015.2015)

There are 285 clients in my network but there are only 128 clients are showing up to date. All the other clients are OUT OF DATE or OFFLINE.

I have gone through lot of forum replies but i was unable to understand the solution. If any one can give me the clear options and steps to solve this issue that would be appreciated.

What i have done?

1. I have checked the connectivity with the client ?

I couldnt check all the clients but check some random clients which show OUT OF DATE status. All of them have connectivity with the SEPM.