I click on full system scan and in most cases nothing happens and sometimes it will scan

Hi,

 

Can anyone help.  I click on the Norton icon and open scans.  I can click  on the full system scan but nothing happens no matter how many times I request.

 

All of a sudden some days later it will then scan. I have tried to do one today because there has not been one since the 18th July I have noticed that quite often there will be a green tick on the Norton icon, what does it mean