Strange. I very often use the Rectangular Marquee Tool (set to remove a selection) to slice off pieces of a larger selection. Especially useful when cropping photos. But of a sudden, Photoshop won't crop the slices off the image. I have to delete the larger selection and start over again. Here's a screen shot: https://flic.kr/p/psA5wJ
. You can see the initial Select All selection, the piece I wanted to remove and the tool used. But when I go to Image, Crop the crop doesn't happen.