Wall Mounting plasma TV..my studs aren't flush with the sheet rock!?

Hello,
I have a new plasma TV and a bought wall mounting hardware. I used the stud finder on the wall and found that the studs are 24 inches apart. I drilled several small holes into the area but it seems like after I clear the sheet rock there is a half inch of dead space before the studs. I haven’t checked the whole wall but it seems this is the case in the relevant area. Is this common? I bough longer screws but I’m still hesitant to hang it on top of empty space. One idea is to use 1/2 inch metal washers, if there is such a thing, to fortify the screws.
Any other ideas?
Thanks