I had to lengthen the homepage on the 320px setting, but when I went back to the 960px page, it was too long.
Whilst grabbing the bottom of the footer on the 960px brings up the correct icon suggesting I am changing the length of the page, I can’t change the length, and the footer does not move.
You have probably got hidden items stopping the footer from being able to move up any further.
Unhide the items using the ‘eye’ icon and if you see any hidden items that are low in the page, then drag them up as far as where you want the footer to be, then re-hide those items from displaying by clicking on the ‘eye’ icon again.