Can't adjust the border when placing multiple images in a grid?

I am importing several dozen photos and placing them in a grid, something I have done many times. This time, I don't seem to be able to hold down shift and use the arrow keys to adjust the border between images; instead it acts like I am not holding shift and changes the number of columns and rows in the grid. Am I misremembering how you do this? Or is the bug in the current version?