Bash history weirdness

[John]

Hi,

Bash's history command is behaving oddly.

If I do "history -w" then it writes the current history to
~/.bash_history as expected.

But if I do "history -a" then ~/.bash_history doesn't get changed, and
from the modification time it hasn't been touched at all.

Any ideas for what might be causing this? Any obvious shell options or
suchlike that I should check?

Cheers,
John