Next clip then back 1 frame

Apologies if this has been posted already.

This has been doing my head in lately. I use the UP and DOWN arrows to navigate on my timeline, but when I push DOWN to go to the next clip, but push LEFT to go back 1 frame (last frame of the clip prior so I can set a keyframe) NOTHING HAPPENS!? It refuses to go back a frame

I feel like this is a recent issue as I've always navigated the timeline like this but it's only just been driving me crazy. Any fix for this or will i just have to cross my fingers that it gets solved in the next update?