Ninebark definitely does best in full sun. Part of the problem with the ninebark under the tree is the roots may be competing with the tree for moisture and losing the battle. I would move both ninebarks to a place with full sun. Water well and mulch. Give them a good watering once or twice a week until you get through summer. Once fall comes along, the roots should be established again and you can back off on watering. Our planting and care guide has additional tips.
Thanks for writing!