Occasionally I get a thin, unsharp, line on one edge of my 8x10 contact prints. This corresponds to a denser line on the negative. It is about 1/4in from the edge of the image. It is on the edge where the dark slide is removed. Usually occurs when in bright light-skies of course- and is not always on images associated with the same holders. Is this coming from declining light trap performance in the holders (they are all less than 10 years old, Fidelity plastic) or from incorrect seating of the holders in the camera back (Canham camera) or are there other potential causes?