"Empty" surfaces

I have started a design which is currently 2D on the horizontal plane. So far it is has been made by rotating and reversing copies of an original tile . After a while some of the tiles showed with surfaces that could be selected and have colour/texture applied and some were empty shapes that could not be selected.
Is there a way to instruct all the shapes to have surfaces that can accept colour ?

Generally if you don’t get faces it’s because there are issues with edges. Either gaps, overruns, or non-planar edges.

How big is this thing?

Share the file so we can see exactly what you’ve got.

Test targets.skp (675.0 KB)

There certainly are overlaps. But I can’t understand why some are ok and others not

Some of it is due to forks in the edges.

You can get faces by closing the loops like this where I added in an edge, shown selected. You can erase it after the faces are created.

You have a lot of incorrectly oriented faces. Make sure you get them all correctly oriented.

OK. Thanks Dave.
I’m thinking that “forks in the edges” indicates that an enclosed shape cant think of itself as a surface if there is a line end within it ?
Also , as both the back and the front of a surface can take colour, would a visually a complete pattern not look the same on differing orientations ? However I get the impression some operations more advanced may cause problems so I’d better not take short cuts.
Thanks again