David
I have the same jig, as do many others, and it really is a nice jig for dove tails.
You need to get that supplemental manual. It contains a lot of info not covered in the manual that came with the jig. Why it's not part of the manual is beyond me???
Make sure that the pressure bar is putting even pressure across the material holding it in place. If you have a relatively narrow piece, you may need to put a second piece of the exact same thickness under the pressure bars.
Right now it sounds as if your jig has to move the tail section forward to narrow the tails. Trying to use the reference markers on the jig is a good way to get close, but I have mine etched. Being off a pencil line's width is enough to throw things off. Use a depth gauge, or the tail of a vernier caliper, to make your jig adjustments. This way you can keep track of how you have been adjusting it, and know that both ends of the jig are in the same plane.
Be absolutely certain that the reference stops are square to the material, not the jig.
Porter Cable makes a centering cone for their bushings. Use the PC Brass, or equivalent brass bushings. I found that the aluminum types had a tendency to loosen up and I could not get them to stay in place as well. There used to be some made of stainless steel, but I forgot who made them, or if their still available.
Next time I do DT's with mine, I'm going to find a second jig also. I use dedicated routers, but it would be nice to have a dedicated jig for each.
It is a trial and error for that final fit. Any change at all and you end up starting over again. When I cut them, I don't touch anything until I'm done.
PS.
Brazilian Cherry, or Jatoba, is very unforgiving. It has to be exact, because it machines like mild aluminum. You will not force a Jatoba joint, or rely on crushing wood fibers, for a "tight fit" with Jatoba. I've used it a lot through the years and you have to work it to almost metal tolerances. It's beautiful when finished, absolutely stunning, but you have to be exact with it.