Patching and Plastering - Patching making causing wallpaper to bubble.

I took two old electric heaters off of external walls in my bedroom and bathroom. It took a lot of plaster off so I have patched it up ok. However 6 months on and the wallpaper I used to paste over it is bubbling and splitting where I patched up. I replaced the wallpaper in the bedroom and it has happened again. The patching still looked fine...nice and smooth and no visible gaps.
I can only think that somehow air is getting through the brickwork and causing this although outside the brickwork looks in reasonable condition and is well mortered.

How do I stop this? What can I use to seal the whole area?

Ive done patching on walls that are not external, then wallpapered and im not having this problem.

I would be grateful if anyone has any idedas.

Je$ter

Did you prime the repaired area prior to wallpapering? It likely is not air, but a failure of the wallpaper glue to bond.