I have the same issue. I recently returned to SWTOR after a long break. When I first logged in, I had 0% achievement completion. I have two 50s, and a bunch of lower level characters. I have completed the story missions on both 50s and done a good number of fps and ops. I didn't get any achievement unlocked, not even the achievement for having made lvl 50 in a class. Tried copying a character to the PTR to see if that would make any difference. It did not, I didn't get any achievements unlocked there either. Hopefully this issue will get fixed soon.