s this was all about the pinkie collapsing and sliding under and back towards the second fret, either on the 1st or 2nd string when playing G/B as per Justin's fingering.
I had another look at the picture you posted and you have the palm of your hand is parallel with the neck
curling you pinky. Try turning your wrist a little towards the headstock, this will straighten your pinky out
so it won't collapse.
Don't hold off on the IM course because I keep distracting you. You can play all the open chord songs using
That's a good way to practice your barre chords, playing songs you already have down
in open chord using barre chords.