Wallpaper and Wallcoverings - Wallpaper peeling away after 30 minutes

My first papering project: I am lining a textured wall before I go ahead with the final wallpaper mural, but I am having problems with the liner.

I washed the walls down yesterday & papered the wall today. All was going well until half way through some of the paper started to come away from the wall. A quick peak below and the paste seems to have just soaked into the walls and there's none on the paper. I gave them another good dab of paste and waited. Some seem fine, others coming away again after an hour.

I didn't prime the walls as the paint seemed to be in good condition, not flaking or coming away. It's a flat paint which I thought would be fine as the paper directions say prime with flat latex paint.

I only have a couple of days to get this whole room completed. Is there any way I can prevent the whole thing just coming down, or am I being impatient? Do I need to just wait overnight? I know when my dad used to wallpaper, bubbles etc seemed to miraculously vanish by morning.

Thanks!

Always prime with a wallcovering primer. The glue just absorbed into the flat paint. I would take it all down, wash the walls to get all the glue and prime first. Otherwise trouble ahead.