Wallpaper and Wallcoverings - Small Intermittent Gaps in Seams

We just had our dining room wallpapered with a dark flowered pattern. The paperhanger we hired came highly recommended and we are generally pleased with him. However, at various seams around the room it seems that there is a slight gap between sheets such that the white wall underneath shows through. At first we thought it might be imperfections in the wall because part of a seam was perfect while farther down the wall there would be a slight gap. Is there afix or repair for this? The pattern is mostly green and dark green with flowers. Are there touch up sticks or something like that? Or should there have been something done before the paper was hung, like painting a green stripe on the wall where each seam would lie?