Multiple image gap size adjustment

Good day everyone, 

I'm having a difficult time finding the solution for image frame gap size adjustment. It was the page up and page down buttons that would open and close the space between the photos frames, but now, for some reason, it is no longer working. I'm beginning to this this is a bug, and if I recall, it was happening in the last, prior to June 2020, update. 

Screenshot attached for reference. 

Anyone have any idea?

Thanks in advance.