Also I don't think there are any DCs currently active that would prevent assists. So most likely either DR, crit immunity, or Deadly Bluff are the culprits. Are there any other kit mechanics that stop assists (exempting debuffs)? I can't think of any right this moment, but I'm not really familiar with the intricacies of the Inquisitors faction.
And on that note, I'm also assuming that there's no daze or anything similar in play, since you stated the "obvious stuff was confirmed not to be the problem".
Also, if it was in the current GC on Ilum, you can't assist or attack OoT as long as you have the "Hesitation" status effect. That can be an easy one to overlook if you don't read the fine print.