When I crop a photo, it no longer fills the screen even though I have set NAVIGATOR to "FIT". This is particularly annoying when I create a slideshow, since some photos are full screen and some are only half screen or less. This makes the slideshow option useless! Why doesn't the ZOOM function actually zoom?
Have a look at the dimensions of your cropped picture. You may have cropped your picture so tight it does not have enough pixels to fill your screen. Not sure that LR will go beyond 1:1 when in "FIT". You can compare 1:1 and FIT to see if there is a difference in size.