which is the curved piece?
If the slabs edges are straight, then I would suspect the strips are curved. That will happen because they are narrow in width, and it won't matter in the big picture as they will be "straightened" by the slabs when clamped up.
If the slabs edges are not straight that should be addressed by jointing or hand planing, or a board straightening jig on the table saw. Then the gaps will close when clamped up.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)