MAC OS/Windows permissions issue

We have a MAC server which we have networked with several MACs and a few Windows PCs also. Up until a couple of months ago we've had no issues running this set up. It's run perfectly with the Windows PCs - they've been able to create folders, add files, edit folders etc and our Macs have been fine accessing any of these files or folders. However since our MAC server ran it's last OS update we now have an issue in that whenever any of the PCs create a new folder or file - our MACs don't have permission to open or edit them. We physically need to go onto our MAC server and go into the files and then change the permissions so that everyone can read or write.

We never had to do this previously and it's causing difficulty getting our MACs and PCs to work together. Does anyone have any ideas as to what we can try to fix this?

Another issue is that when our Windows PCs are saving things or using things on the server it is incredibly slow - snails pace - which again was never an issue previously.

Sorry it's not a very technical sounding post, I'm not a tech guy, so just trying to describe it best I can.

Any help would be greatly appreciated.

Thanks