

It’s not uncommon for vinyl wallcoverings to develop bubbles as the paste dries. This is called “off gassing” and it happens because the vinyl is impenetrable and there is nowhere for air to escape.
But these days, most paper products – such as this one – allow air to pass through, and dry nice and flat with no bubbles or blisters.
One thing to keep in mind is, virtually always, these bubbles will disappear as moisture evaporates and the the paper and paste dry and shrink taught. So the less fiddling you do with it, the better. (And exception would be large bubbles, which should be addressed.)
One way to treat bubbles is to chase them out toward a seam, using a tool such as the plastic smoother. You can also use a pin or razor blade to poke a tiny hole in the paper to let the air escape. It helps to do this in a part of the pattern, and not in an unprinted area, to disguise the hole.
I also like to understand why something like this happens. If you know what caused it, you have a chance of preventing it the next time.
I primed the wall with my usual primer, so I don’t think there would be issues with moisture being trapped between the primer and the wallpaper. But then again, you never know what is inside the wall – many walls have been painted multiple times, and some layers may not be compatible.
Another cause could be the plastic scraper. The bristled smoothing brush uses a more gentle action to push the paper into place. The scraper could actually overwork and stretch the paper. In fact, if you use the scraper to try to push bubbles out to a seam, you end up creating more bubbles.
I use the plastic smoother mostly for seams, and not so much for smoothing a full strip into place. But it’s possible that I used it more than usual this time, and stretched the paper, creating conditions for the bubbles to form. In fact, on one strip, I gently pulled the bottom portion away from the wall and then put it back in place, but making sure to use only the bristle brush. This did eliminate the bubbles.
But, again, if you just wait it out until the paper becomes thoroughly dry, nine times out of ten, all the bubbles will be gone by morning.