Flex Agents as part of standard Flex Orchestration are removed from Chat Channels once they are done working a specific Task related to a Channel. Possible cause to this error is that this orchestration is not working or Flex is unaware that the Custom Task Channel used has chat capabilities.
Please check that Agents are not part of large number of inactive Channels/Tasks, review any custom programmability you may have in Flex UI (please review Flex Documentation). You can control the Chat Channels and Members through Programmable Chat API. You can also change the Channel Limit in Twilio Programmable Chat Console but keep in mind this cannot be extended indefinitely.