Instanced meshes have poor shadows

I have a collection of instanced static meshes that were populated using a BP. Their mobility is set to ‘stationary’. For some reason, the mesh instances are not producing shadows or have very weak or poor looking shadows. Placing one the meshes as a normal static mesh actor with stationary mobility produces a good looking shadow. The scene is lit by a single directional light (also set to stationary). Why are the instanced meshes not wanting to make shadows?