I'm having the same problem. It accured after the 3.1.3. update. The lift and Stamp tool are busted.
I click on a image to Lift and I hear "warning tone", and it WILL lift but the ARROW STAYS pinting up. Then after I CANT automaticaly stamp on anything.
Before the Udate there was no "warning or error tone" during Lifting.
Before the update, the ARROW would aoutomaticaly point Down after lifting and was ready for stamping.
I already uninstalled and re instaled Aperture.
Im running Snow Leopard
I already changed my Keyboard commands around.
I already unplugged my computer and monitor.
I already moved my mouse and all of my connections to different usb ports.
Something is wrong here. Something has changed and now Aperture is broken.
Apple please fix this. It is slowing down my workflow a lot. This is my work program.
Frank T
Aperture_3.1.3_update_lift_stamp_tool_problem_wont_stamp_apple_help_lifting_tone