Loose electrical connection AFTER wallpaper is up

Ok, I got the wallpaper up yesterday morning (finishing the lower
floor), and I discovered hours later that there's no power to the
upper story. The coach lights still turn on just fine, so that means
the loose connection is AFTER them. But there are two splices between
the coach lights and where the tape wire emerges out the stairwell.

How do I fix this without having to pull the wallpaper? And is it
always like this? I'm finding the electrical to be kind of flaky. Is
round wire more robust? It seems like it might if all splices are
soldiered.

Cathy Weeks