A block isn't showing on my page

The three usual causes, in order of likelihood.

1. 1Check for validation errors — open the block in the editor; a red hint means a field needs fixing (usually the URL). Invalid blocks are hidden from visitors on purpose.
2. 2Check it is toggled on — a switched-off block stays hidden.
3. 3Check the page is active — a deactivated page returns not-found to visitors, blocks included.