So I was having the same problem as you. You might try ignore camera zoom under the parallaxbackground parent.
The problem that I was having, and it kinda sounds like the same problem you are having, is that mirroring in parallaxlayer only mirrors the image once, by the x and y parameters, which should be the same as your image size.
Now the problem for me was, my image (a simple sky), was too small, so while it is getting mirrored correctly as I move through the level, it was only filling about half the scene, since the image I was mirroring was so small.
The reply below from somebody else suggested changing the scale of the image, which is what I tried initially and .... is fine, but kind of a crappy fix since you lose the resolution of your original image.
What I did which worked well, was just to make the image bigger. I opened it up in krita, tripled the canvas size on the x axis, then copypasted the original image twice, lined the three pieces up in the bigger canvas, merged the layers, and boom, now I have a sky background that's 3x as wide, and easily fills the scene as it gets mirrored.