Why some of the 'tiles' have no images

[dtsig]

I just clicked on TV and it came up with two sections. Recently watched and Recently added. Not sure what controls this but let's move on.
There are 6 'tiles' in each section. In the recently watched section 4 of the 6 'tiles' has the image for the series as found in the individual video (artwork) and in the folder. In the recently added there were only 2 of the 6 'tiles' that had an image.
In all cases the files and folders have images.