[fetchmail] fetchmail: removing stale lockfile

I have setup fetchmail to run every 10 minutes using CRON, to fetch my emails 
from multiple accounts and deliver them to the local server (exim). This was 
all working perfectly, until recently. Now whenever fetchmail runs it generates 
the message 
"fetchmail: removing stale lockfile"
(Which cron sends to me as an email). This happens every 10 minutes. It also 
doesn't seem to be checking at least one of the setup accounts because of this.

Does anyone know what this message means, and how to fix it?

Thanks in Advance,
Luke