users on realms don't see my chat messages

Hi! I searched in Google and on the forum, but unfortunately I did not find any information about this. We have our own realms server for 10 people. It exists recently, we haven't changed anything particularly in the settings, but we ran into a problem. Some users write to the chat, but their messages are not visible to others. Such a problem appears and disappears, and each time it happens to different server participants. This really interferes with the joint game!