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.