I've been meaning to write this post for a while -- started it several weeks ago, in fact -- but life keeps getting in the way.
One of the first things I built that was useful was a bench hook. It's a simple device: a piece of 1x8 scrap, with some scrap 1 1/2" square cedar for a fence and the hook. It's always been pretty good as a bench hook, but this summer I started using it as a shooting board.
And it worked OK, except boards were never quite square. So I fiddled with it. I checked the base for square: it was fine. I made sure the fence was square: it wasn't, but I fixed that. I checked to make sure the top of my bench was really flat, and that THAT wasn't throwing things off. Nothing worked.
Finally, one day, driven to the point of distraction by this oddity (the plane was cutting nice shavings, it just wasn't leaving the edge of the workpiece square to the faces), I realized there was one thing I hadn't checked. So I checked it. And, as you've probably already guessed by now, my PLANE isn't square. In fact, the side of my nice old #5 isn't even flat. The thing rocks, somewhat at random, which is why I couldn't get a consistent cut.
It's not worth replacing the plane for, and I didn't feel like trying to flatten it, but at least now I know, and I can make allowances. Or just use a plane that IS square.
So remember: if your jig doesn't work right, the problem might not be the jig.