I think it's because your not doing pixel rounding for either/both the background and camera/scrolling.
If you don't want to do pixel rounding, have each sprite use an extra 1 pixel on the right/bottom side so that the graphics overlap.
Also if you're using any scale/zooming etc., always make sure you have 1 pixel transparent border around every single sprite (pressing the 'crop' button in the image editor will usually do this for you). Otherwise you get bad looking edges, which could maybe be the problem in this case.