Are the Characters that are being left behind sending a message like "Hey! I got left behind please run me to ChaseCharacterName"? Chasing characters will drop out of the chase routine when they have agro and are with in CampRadius of who they are chasing, or the character they are chasing get's to far away, either CampRadiusExceed, or Zdistance >100 away. There are times when a character will be casting a spell that takes a long time to cast, will be left behind.
For time when you have characters that don't make the trip with you. You can always use the /trackmedown command to get them to run to the chase character. From the chased character, I use /bca //trackmedown to send the command to everyone in the group.