After a number of intermittent and unexplainable issues with active user devices not syncing, I see that IBM has issued a fix in IBM Traveler 188.8.131.52 and later releases.
Once a user reports the issue, you can confirm by issuing the following console command:
tell traveler user "john smith/org"
The output will include:
Traveler: Monitoring of the database for changes is disabled.
The temporary workaround is to issue the following console command:
tell traveler push enable "john smith/org"
Now the “tell traveler user” command should return:
Traveler: Monitoring of the database for changes is enabled.
..and the user’s mobile device will begin to receive the backlog of messages again.
To resolve the issue install IBM Traveler 184.108.40.206 or later and cross your fingers.