What if you fully open the valve?

If it is only skipping on upstrokes, maybe the nib gap isn't quite wide enough and so it isn't flowing ink under low/no nib pressure. I have noticed on some pens with this problem that widening the gap very slightly solves the problem.

Does the pen leave a line on the page if you hold it by the tail and draw it across the page with the nib resting on the paper? (that is, with nearly no pressure on the nib) If not, it isn't flowing enough.

Baby's bottom may also be at play.

Iroshizuku Take-sumi may also help as it tends to flow better (than even Quink Black in my experience)