crontab

Realy 5 minutes job. I have a crontab set on server but I don't know why it doesn't run. It should run every 4 hours how it's set now, but it doesn't. What I want to do is run an update on a page on the server every 3 hours. If someone can fix this and tell me how to do it for free it would be great. I will give you a 10 review for this job, but I'm not looking to pay any money. Please bid if you are interested and agree with the conditions. Thanks