I haven't used one for years, but I seem to remember we used to have to dial in the thickness of the planed material just right to get the joints to fit correctly. Doesn't make a lot of sense, particularly if you're using composite boards. And make sure everything is clamped in the jig nice and square, and flush where it's supposed to be.
Also make sure you are running the guide collar right up against the fingers when you cut, maybe take a second pass through. Make sure the bit you're using hasn't been resharpened....Take pains to be sure the bit is centered in the collar.
If you've done all that, I'm afraid you just have to play with it until you get it to work...use some test pieces and you'll see that the depth setting does affect the fit, but they're not as versatile as an infinitely adjustable jig like a leigh. I'm afraid I'd have to put my hands on it to remember and see what wasn't working...
I hope that may help in some way or another...
Here's a link or two that might help as well:
I hope this information allows you to resolve this issue. If you need further assistance, please post back with a comment to this thread.
If I've managed to answer your question or solve a problem, please take just a moment to rate this post....thanks!