Hello bgomes,
Thank you very much again for your response.
There's only one difference between the sample script in the link you introduced and our script. We have an additional line that is "_gaq.push(['_setDomainName',..." since we use a subdomain for our Marketo Landing pages. I am not sure if this line causes this kind of issue. Also, we didn't put those script before "</head>", so we moved it to the right place.
If you come up with other possible causes, I would appreciate to hear it.
Thank you so much,