Is you site getting activity?
How do you have your CRON jobs set to run?
ACP->Setup->Options->System & Performance-> Job Run Trigger
If you have it set at activity, and your site is not getting much activity then it's not going to run the CRON jobs regularly.
I personally have it set up to run mine via a server based trigger, and have set up a server crontab job to run once a minute.
By the same token, if you chose the server based and did not set up a server cron job to run, you will get the same error.