Few of the SEP clients are out of date /not getting virus definitions

Hello ,

We use SEPM 14 version to manage our clients. Most of the time 99% SEP clients are up to date with virus definitions. But every time we get an email stating few of the client are out of date and those clients are random .Even though those clients are showing connected to SEPM with green icon .We usually use  smc -stop and -start for some case it may work else we need to reboot the server .In order to avoid reboot we use the Intelligent Updater files  and resolve the problem .Do you have any idea why  few  of the clients failing to  download the virus definitions.Those clients were  downloading /updating definitions from SEPM without any problem .Can anyone share your experiences .

Thanks

Sujith