This occurs either thanks to the absence of fashion sense or because of the fear of the groom to go screwy in his dressing. Its even gone as intense as having a groom stop off in the store in his suit on the way to his marriage sheepishly asking for our help in tying the, damned piece of clobber. Some cravats come pre-tied but the more genuine ones need to be assembled and it can be a jigsaw puzzle. You then wrap the wide end round the narrow end till the beginnings of a knot forms. You then put the wide end thru and the knot should have absolutely formed. By placing your fingers on the knot you can slide it up toward the shirt button by tugging delicately at the shorter narrower piece of the cravat. When it is flush against the neck it's time to apply the final touches.
Just like a windsor knot it can be sweet to squash the knot on the slide up and this could have quite the effect of making a little kink that makes the shape look much better.