Welcome abroad Chicki.
I once worked with a guy by that name (nickname of course).
This convoy you're pursuing, do they all start to zigzag or do they all just veer off and continue on a new course. If it's the latter, it may simply be that you caught up with them at an ambush point where they just happen to change course. I've had this happen on occasion too. Just try to set up an ambush point that's along their new course, or sooner (if possible) on their previous course.