Can someone tell me what I'm doing wrong?

I'm trying to rescale a selection. I have a single-layer image 640x640, I've cropped off some bits I don't want, and then I want to resize the cropped selection back to 640x640, stretching it somewhat.

I've selected the cropped image, clicked the scale tool, ensured the scale tool is set to 'transform selection', clicked on the selection, written 640x640 in the size box and hit the scale button.

Nothing happens. The image is still its original size and refuses to stretch to fill the layer.

I've tried altering various parameters in the scale tool, to no avail; the image just will not stretch!