Not a solid object - how to fix for subtraction

I thought I had made this solid but apparently I have not. This was all done with extrusions. Could someone help show me what I missed?
Wheel Lock.skp (172.3 KB)

I’m not sure but I think you need to delete the faces half way down the holes. If that’s incorrect I’m certain you’ll get a correct response.

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spot on @Guido

I grouped it, ran solid inspector, and lo and behold, there was a face inside.

now it’s gone, it’s solid.

Wheel Lock 2020.skp (185,0 Ko)

Glad it worked!

Thank you. I don’t know how I forgot to run that