Quickie.._visible. why isnt this working?

hi,

ive got a load of movieclips on the stage that i want to hide/reveal at certain frames.
This is the code ive applied to the frame:
stop();
_root.resitab_mc._visible=false;
_root.officetab_mc._visible=false;
_root.leisuretab_mc._visible=false;
_root.urbantab_mc._visible=false;
_root.officefitouttab_mc._visible=false;

but none of them are hidden!!

whats wrong? im sure ive done this before.