Fixing plaster cornice to walls

Just bought new plaster cornicing from homebase, the kind you glue onto the wall / ceiling, however when I got home I discovered that of course my walls aren't flat so when you go to put the cornice up only parts of the surface that you glue to the walls and ceiling actually touch the walls and ceiling.
I'm worried that given that only around 50% of the surface that should be glued to the walls/ceiling actually touches the walls, that the cornice won't stay up - anyone any experience of this or know how I might go about fixing them to the walls?

Thanks in advance

you can buy a adhesive filler, evo stick do one in a black tube, dry's white , supposed to be the strongest grab you can buy, i used it to fill in around my cornicing where there were gaps.."just in case"

I draw a line on the wall where the cornice will be and every few feet, I put a tack in the wall for the cornice to sit on. When you put up the cornice, put tacks into the ceiling too. This will support it until the adhesive sets. When it's done, just take out all the tacks and fill all the gaps with caulk. I don't know if I explained that well?

I screwed mine to the ceiling/wall by making a hole with a drill bit just big enough to sink and hide the head of a plasterboard screw, then covered over with caulk. This gave a very good cosmetic result and is very quick to do. I got the tip from a builder friend.

Thanks for that - I didn't know that you could drill into solid plaster - any particular drill bit to use?

The adhesive filler could also be a good idea - I might try that.

Thanks

Using a small nail or tack to support the coving is how the pros do it (as Jackson suggests) Coving won't be subject to any loads so it's not critical to get a full bond all round the room. As suggested fill any gaps with the adhesive.