Maybe this is your issue. When JTAC targets a group containing multiple enemy units, he will read out coordinates that describe the approximate location of all members of that group instead of coordinates for one specific enemy. For GBU employment this should not present an issue since the laser will designate one particular unit, but using these coordinates for JDAM is unlikely to result in a hit. To work around it use separate FAC-Assign Group commands for each unit you wish to target.