Panellists unable to respond to attendees

Hey! We host webinars which involve multiple speakers who join the session as panellists. For the chat, we set it up so that attendees can chat with hosts and panellists, and panellists can chat with everyone. 

We've recently found that occasionally some panellists are unable to directly respond to messages from attendees, with all attendees' names being greyed out. This issue doesn't seem to have any rhyme or reason, with panellists being seemingly randomly affected by it - they can join the webinar one week and be fine, then find they have the issue the next. 

Does anyone have any suggestions as to what might be causing this?