I had the same problem in my last house, where the plaster was probably 30+ years old and in one bedroom in particular where there was some damp. I borrowed a steamer and the plaster literally fell off one whole wall, which I had to get totally replastered. This happened in a few places around the house where as the previous poster said the plaster was unsound, so I assumed it was better to have it replastered anyway?
I quit using the steamer at this stage, and on the better walls, used to dampen the walls first with just a sponge and water, then splice the wallpaper in a criss-cross pattern with a stanley knife and then remove the paper with my hands or a scraper. It takes longer but was less destructive. Having said this, I still think if the plaster is unsound there is no point in papering over it, better to re-plaster and let it dry before you put new paper on.