Plasterboard wall ceiling problem - studs or joists seem to be set back from wall??

[ZAZ]

right so i've got 12 10kg basstraps to mount (each one is 120x60x10)

have got a stud finder to scan the plasterboard walls and ceilings - scanned the walls and it found studs and joists at even intervals

drilled into the wall in two spots where a stud was mean to be - there was nothing there behing the plasterboad (which was very thin) - put a 3 inch long carpenters awl through the hole and the entire 3 inches went in before it tapped something solid - experimented with drilling a hole at a point in between two studs and exactly the same thing, the awl went all the way in before tapping something..

mounted one trap on the wall using rawlplugs and two hook screws- it mounted fine -

but i'm worried about the ones that need to hang in the corners between ceiling and wall - the plasterboard seems so thin - the ceiling is one of those with lights sunk into it ( i duno whether this means that they have hung a ceiling below the original ceiling in ordder to have these implanted lights) -

any suggestions?

[spm_gl]

You definitely have a suspended ceiling. If might have joists somewhere. Why don't you remove one of the lights and look inside with a dentists mirror?