Are you in a layer view, not a comp view?
Are you using After Effects version 4.0? 5.5? 11.0.4? 220.127.116.11?
Mac or Windows?
What, exactly, are you trying to get AE to do?
What, exactly, are you doing as you attempt to get AE to do that?
And answers to everything mentioned in Dave's link.
Also, Export? I don't know of anybody who uses Export.