Search block will not appear

At first I thought it might be a theme problem, but I have since tried all the other themes (still doesn't appear) and all the other blocks (they all work fine); I have not modified any of the files outside of my own tweaked duplicate of Garland.

The block administration interface shows the Search block as being there in the left sidebar (or anywhere else I chose to put it), but it simply doesn't appear. Looking through the source there is no sign of it - neither is there any indication of an error.

Any ideas as to what might be causing this?

^{PS: I posted this in the issues section of the site. It gave me the option to state I was looking for support, which I was, and of course it's an issue... But then I realised I couldn't delete or amend that post. Should I have made it? Can I get rid of it? Etc.}