That's (unfortunately) your issue. there was a time when higher level players were going into the lower level Fleet Action instances, to the point that no one at the proper level for the instance can do enough damage to get on the scoreboard; SO, as a 'temporary fix to be revisited later'; they level banded the instances. What does this mean?...
Lts. can ONLY be in instances with other Lts.
Lt. Cmdrs. with other Lt.Cmdrs.
So... if your character is an LT. and the person you enter the Fleet Action with a friend who isn't (say he's a Captain); you'll both be shunted to your own instance - and the only other player that enter the instance with you will be Lt.1 to Lt.9 and for your Captain friend, the only other players that can enter his instance must be Capt.1 - Capt.9
I treally hope they get back to the Fleet Actions in General and figure out a better solution than the Level banding that's still in place.But again, if you're all different main ranks - that's why you don't all go into the same instance.
Ohhhhh that explains a lot. ( NOW ). Would have been a helpful thing for us to know in game so we didn't waste so much time trying to get our team into the same instance.
Another quirk with this misison. You're an LT when you get the misison, but it won't let LT's into the misison at all. It simply tells you that you're too low level and to come back when you grow up. If LT's can't do the misison why give the misison to LT's?