Kerning Pairs...Ahh

Sorry to post, Im just having trouble kerning in Glyphs mini.
I typed in the two letters I want to kern, placed the cursor in between them, and altered the ‘kern’ value in the grey box. This has worked for A, But as I’ve got to B nothing is changing for the ‘BC’ combination, even if i after the kerning number.

Not sure what I’m doing wrong?

In the picture, the kern value is for the right side of the C, You have to kern the the left side in this case.

I’ve figured it out now, thanks.

But still when I enter a + amount into the kerning box, I have to then re-enter it into the kerning window on the right.

It works fine for - values… weird?

Can you make a screencast so I can see what you are doing?