I set up 5 Trucks. Didn't change their formation, so they were in a column.
Fired a SLAM at them from 63 nm, HIGH/SKIM profile.
Dead on hit on the lead vehicle, destroying 3 of the 5 trucks.
Passed the target, and turned around. Set profile to MED/POP and fired when at optimal launch point at about 36 nm. Again, direct hit on what was the lead vehicle. No additional damage done to the surviving vehicles.
Looks like they were working... Dunno that happened in your case. You sure they didn't have any waypoints so that they could have moved slightly?