your INS drifts over time, it's a miniscule amount but it's big enough to affect weapons, what you have to do is enable HNS after FAST align if GPS is available or do a full GC align if not.
another thing that might be causing the weapon's INS to go bunkers is a long straight flight, if you continue to miss even after enabling HNS, do 2 90 degree turns to fix the weapon's INS.