I'm guessing that you may have a board orientation problem. One of the things that can happen when using a dovetail jig with an adjustable finger pattern is that it is possible to set the fingers to a nearly identical spacing, but one that is not exactly identical when one end is compared to the other. To the eye, both ends look identical, so it's easy to get confused about which way the pins or the tails were cut. Have you tried turning the tail board over (inside face to outside face) to see if the pattern matches and fits together in this orientation? Board orientation is critical and it's easy to see which orientation is correct if the pin and tail patterns are varied in size from one end of the joint to the other. Leigh provides a marking system in their instruction manual that, if followed, will help to keep the boards correctly oriented during the cutting steps. If not followed, it's easy to cut a nearly symmetrical joint that won't fit together, similar to what you are describing.