The problems and causes:

Wallpaper Air Bubbles

Home of the Bubblerepair wallpaper paste syringe kit!

Fix those annoying bubbles and lifting wallpaper edges with ease! All profit goes to charity.
DIY Doctor
Handy guides on how to wallpaper:
1. Poor hanging technique where the paper simply hasn't been brushed down firmly enough with a paper hanging brush to expel all the air. In this case, if the paste underneath is still wet, you might get away with a tiny knife incision in the paper to release the air without having to re-hang the sheet.
2. Emulsion over lining paper can result in moisture from the paint working through the paper and into the paste below. If the bubble is actually wet paste rather than air, it will disappear again as the paint and paper dries out. If not, its probably an area of paper that wasn't stuck very well and the moisture in the paint/paper has helped detach it.
3.  Missing bits when pasting, or handling edges which can result in areas where the paste has been wiped off and these may lift over time.
4. If the paper goes over sources of heat (e.g. hidden central heating pipework), expansion and contraction may over time cause paste to fail and edges or sections of paper to lift.
5. Not "sizing" the wall properly - especially when papering over fresh plaster. Personally, I prime fresh plaster with breathable emulsion anyway and leave it for a while to dry out thoroughly before sizing, lining and emulsioning.
6. Not allowing the paste to soak into the paper for long enough.
Click on the Repair! button for details of the Bubblerepair kit to fix bubbles and lifting edges!