I may be wrong (correct me, anyone), but I believe you can have a "sliding hernia" that moves up and down, and therefore might not be noticed in a random barium swallow.
I also personally think that sometimes the barium drink they provide is very thick and doesn't really act like the acid contents in your stomach that might be bothering you at night. (again, correct me if I'm wrong).
Hang in there. I had many tests, and it took many years before I was FINALLY recommended a consult with a surgeon.
Hopefully you can get feeling better without surgery...still, persevere! Also, I'm sure I've read of at least one member who had a Nissen but didn't have a hiatus hernia.