I don’t know your dataset but If you cannot collect more train data to cover your validation distribtuion you can try with some interesting augmentation approach like:

  
      

      arXiv.org
  

  
    

PoseAug: A Differentiable Pose Augmentation Framework for 3D Human Pose... 3

  Existing 3D human pose estimators suffer poor generalization performance to
new datasets, largely due to the limited diversity of 2D-3D pose pairs in the
training data. To address this problem, we present PoseAug, a new
auto-augmentation framework...