Featured Post Not Displaying

Everything was working fine … and now it’s not.
My slider image WAS visible and then it just disappeared.

– I created a new post, with a new slider image, and a new category.
– Set Homepage Settings/ Homepage Slider to new category.
– Still didn’t work

In Posts, in both of my slider posts (I only have 2; the original and the test) the Featured Image DOES NOT APPEAR in the bottom, lower corner. It says ‘click to edit or remove’ – and I do so and I select a new image, but the thumbnail image doesn’t appear, to confirm that the Featured Image has ‘taken.’

When I view the actual post, no image shows up.
Please advise.

Note: I updated to Adamos 2.9; I am working with a child theme.