I had the same problem when I fitted my neighbours living room wall paper there was a big bubble in the middle of it I just kept slapping it untill it went flat that seemed to fix the problem .then my neighbour asked me if I had saw her budgie.
joking aside cut the bubble with a blade stanley knife or something sharp then apply paste inside bubble onto the back of the paper let it soak for a couple of minutes then gentley pat the paper onto wall wipe off exsess paste that will come onto the surface with a damp cloth .this will fix yir bubble.
ALEX FALKIRK., September 2008