Apache error log stops updating on some virtual hosts

This is a strange one. On Ubuntu 10.04 server Apache 2.

Yesterday my error logs stopped updating on just some of my virtual hosts. But it is updating ok on other virtual hosts so I know Apache is set to output errors to logs.

It was working before, then 3 virtual hosts stopped updating errors at the same time.

The access logs are updating ok.

All the virtual host have the same format in the VirtualHosts entry in Apache:

ErrorLog /srv/www/mysite.co.uk/logs/error.log

I only thing I was thinking was yesterday I made 2 changes:

1) I issued a tail -F command to watch some of the logs.

2) I added a new virtual host for SSL and installed a new SSL (which is working fine)

I have tried restarting Apache and even rebooting the server.

Any ideas? It is driving me mad!