This is going to sound counter intuitive but it worked for me doing cables.
When you insert the right needle into the stitch, before the gap do not stretch it, after the gap stretch it as far as it will go for about 3 stitches and then slowly stretch the stitch less and less until you get to the end of that DPN where you're not stretching at all.
The reason tightening the stitch you are making doesn't work is because the problem is on the rows below that stitch. Since the problem remains below, the flexing of the gap allows the stitch you did tight to pull loose at that point.
Do that in addition to what Jan said and it should help greatly.
You can poke, push and pull around on those that are already ladders and make them look better.