Hi @Claudia_Locsin,
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.
Did this work for you previously? If so, was there any change to your setup around the time this started occurring?
As @wizardofoz suggested, a reboot of the Core machine is definitely a great first step.