reason for a outbound thread not try to send the message out

I have a situation that a outbound thread didn’t try to send the message out.

In log file, I can tell the message was received from the inbound thread, normally the log would show the thread try to send message out, but for this message it doesn’t.

unfortunately we cleared the recovery database and error database.

I checked with other resources and learned if the connection is down then the engine will not try to send the message out.  but I don’t see any connection error at that time from the process log file.

is there any other logs/files I can check for the connection status at that time?

or is there any other possible reason the thread is not trying to send message out?