Arrows don't always work

Hi guys,

I always view, adjust and crop my images in D against a white background by pressing L twice.
Works great for me.
But when I want to move to a next or previous frame ... arrows fail. More so when I am in full screen. Is there a reason - and a remedy? That would save me the time from going back to a regular screen to click on previous or next frames.

It's been dogging me for years but was always too lazy to deal with it; or too pressed for time.

Thanks a lot.