Sorry this is happening. I'd be very displeased, as I'm sure you are. This might help:

From Bob Vila forum

Thanks. My biggest question is why would the seams first start showing 5 months later?

It could be happening because of the change in humidity that comes with a change of seasons or normal house settling. If your wallpaper guy is a professional, he shouldn't have a problem with coming back and taking a look. Ida's link has some good info.

Thank you. This is the info I needed before contacting the paper hanger.

FYI, here are 2 photos that show the problem:

She also said it might be possible to dye the seams. Is that something reasonable to do?

Paper hanger just got back to me after looking at the pictures and said that she thinks it should be fixable (whew!) and she will come to take a look at it.

I had this happen to me the last time I wallpapered my DR. It was a "paper -paper" - no vinyl coating etc. I had hung the same paper in a different color myself previously and had quickly learned that this paper would "shrink" as it dried, so instead of butting edges, it had to have a slight overlap. When it dried and shrank, the edges were butted perfectly. But he was a "professional" and told me he knew better and ignored my instructions. Of course, one can now see a white line on the dark bottle green tone-on-tone striped paper. Not fixable except to "chalk" the white line. I have yet to find the right color of chalk and just have to know that I'm probably the only one who notices it. But it was an expensive Cole's paper from England and is now only available by special order at great cost so I've been living with it now for about 15 years. When I decided this winter to also paper below the chair rail (had enough left to do this), I insisted that the new paper hanger follow my instructions. He did so VERY begrudgingly but admitted when he finished that it looked perfect. Victory!

Mine is also paper without a vinyl coating. I sure hope she can do something.

Probably not helpful to the original poster, but having hung miles of wallpaper, paper type and vinyl and vinyl coated, I have 2 tips. (I refuse to call them hacks ;-)

One tips, as Anglophilia suggested, is to overlap one edge slightly. The overlapped edge should always be the one facing away from the most prominent source of light in the room.

The other tip is to figure out in advance where the joins will take place and paint a several inch stripe of a color the height of the wall, matching the background color of the wallpaper at that join area. This will allow for expansion and contraction of the walls. Paper type wallpaper is the most problematical in this regard.

Wonderful, and it's so nice to hear of someone who stands behind their work.

Thanks for sharing. This is useful information to know, especially for people who are considering using wallpaper again now that trends seem to lean in that direction.

Yep, paint will definitely do the trick. I've had wallpaper in many homes with myself doing the 'installation'. I've always overlapped since I was aware of the 'shrink' which could become a problem at the seams due to a variety of reasons, such as the settling of a home, as well as the changing in weather conditions. With a white or light background paper, the problem isn't as major, but those dark backgrounds become 'eyesores' and paint was always my solution. Sounds as though your paperhanger has it handled.

Overlapping sounds so reasonable, but it always a bad idea. I don’t recommend that as a solution at all. In some cases the edge of the paper is what is showing. No amount of overlapping is going to cure this. If this is the issue, I have had success using artist chalk to improve the seams. Sometimes the problem is poorly manufactured wallpaper, and this can be the problem even with “expensive “ wallpaper.

What kind of paint are people using to hide the seam? My wallpaper is black matte, vinyl, with fleece backing and is in bathroom.

The best way to prevent the seams of a dark paper from showing is to paint the wall the ground cover of the paper before papering. That way, if the paper shrinks due to lack of humidity, etc. the seams will be less obvious. A very old trick to prevent the seams from showing.

It is the white seam of the wallpaper showing, not the wall underneath. I ended up using a sharpie and colored pencils with great results.

Do you have the contact for the wall paper hanger in Chicago?