Hello @Nigel_Lloyd ,
Welcome to the forum and apologies for the delay in getting back to you here.
So we can better assist you, please provide a brief description of your current setup using this link as a guide.
Make sure to describe your network configuration/topology, including any networking hardware currently in use, so we can have a clear understanding of how your devices are communicating.
Is there any change in behavior if you reboot your Core + Networking Gear?