NTP Server

I’m trying to configure a Mac on our network to provide NTP service to the rest of the network. I have tried following the instructions at https://macmule.com/2013/12/15/how-to-use-osx-server-as-a-time-server/. Initially, clients and network devices can query the IP address of the Mac and receive their time update. Furthermore, the time is accurate. However, at some point, network devices say that the NTP server is down. After deleting the server from the network device and adding it back, it still says down. I go through the process above again and the server is up again but does not stay up. Can someone help?