You need to increase the lightmap resolution of your meshes. More info here:
Beginner here, having problems when I build the lighting. I’m using free low poly assets (Four Seasons - A low poly pack for UE4 and Kenney • Nature pack) - the problem is some assets cast shadows and others don’t. When I move the ones that don’t cast shadows in the preview their shadows appear but disappear after light build, although some remain as a weird blob.
This shot is from when I move stuff around in the preview
And this is after I build the lighting -
Any ideas?
You need to increase the resolution of the mesh that is receiving the shadows, which is the ground mesh in your case. 128 would be fine for the tree but the ground would probably need 1024. Basically the larger the mesh, the higher the resolution needed for nice looking shadows. Of course except for situations like a huge mesh being placed at a location where the player wont get too close, in that case you can go lower.
Thanks for the reply Jacky 
I increased the lightmap resolution of this tree to 1024 but the shadow still looks pretty crappy.
Is there an upper resolution limit that I should expect to see a performance hit or can I just keep increasing the resolution for all the affected assets until the shadows look more like the preview?
Why does a lightmap resolution of 64 for UE4’s included assets create a high quality shadow in the same scene as with the other assets I’ve imported?
That’s solved it, thanks so much