For those of you having a hard time in getting Mo Van Barr to appear, then a possible solution has been found on the GameFAQs forums which will help you in your mission to finally get the bounty once and for all.
According to the post, you will HAVE to start a new game (GULP!). You will then have to play the game as normal by completing all available missions from Bonnie BEFORE doing any other missions.
Once you are done with Bonnie, you can then head over to Armadillo and do the mission with the Marshal. After this is completed, you will have to KILL Walton. Do not disable him, kill him then Run back and complete your mission.
Apparently, this is the only way to fix the bug regarding the Mo Van Barr Bounty, at least until a patch comes out.
Did this work for you? Do you have any other solutions? More information on this can be found on GameFAQS where a host of RDR gamers are getting stuck in to the mystery.