Files are not Read only.. when someone opens an already opened file from same location

I am facing a problem with read only files. When ever I open any Solidworks file it doesn't open as read only. But at that time someone is already working on same file.

As a gropu of team member, we keep all files on server. i am surprised, it was not before i started using SW 2016.

Can anyone help me out.

SolidworksAssemblies