I only read sandworms once, so other information regarding norma and her knowledge/abilities I may be lacking, but didn't she say that she didn't know where to find Omnius? That would explain why she didn't do it earlier, because she didn't know where to look.
Of course, the circumstances surrounding it make no sense. She seems to be pretty much omnipotent, and can travel throughout dimensions, and has had 15000 years to explore the cosmos. In all that time, she never caught a hint, a glimmer of Omnius? Okay, the universe is a big place, so if Omnius had left the galaxy, I can understand that she couldn't find him, but only IF she didn't have super powers, and was more like some random human who was flying around the universe checking shit out. Maybe someone who read the BJ trilogy and hunters/sandworms more thoroughly can try to explain a little more clearly the known extents of Norma's powers.
Disclaimer: This is by no means my way of saying the books make sense. Of course a lot of things make no sense and certain elements written are really dumb, but I thought it only fair to give credit where credit is due, and given my current knowledge, I think it's only fair to consider this.
Of course, if BH/KJA's writing says otherwise, please shoot me down
