I find that the three keys to keeping the harness on the shoulders are:
Proper harness tightness. Too loose or too tight will allow/cause the shoulders to come out.
Proper chest clip placement. A little too low and the child can wiggle out.
Harness as close to the shoulders as possible.
DS is in a TF and as he grows I take the infant insert out and put it back in again to keep the harness as close to his shoulders as possible. When it starts becoming a problem, I put the insert in (or take it out, if it was in), adjust the harness height if necessary, and it's much better.