Internet Access Issue.

We have a Fortigate 210F, we want our LAN user to access the internet, we have added a policy and static route. Unfortunately the user can access some webpages and the other webpages either blocked or the content can not be uploaded completely.

for example the user can access www.google.com, facebook.com. at the same time twitter.com webpage can open but not all of its content appears, while outlook webpage can not open at all. the policy is like this:

source: All Lan user

destination: All

service: All

NAT : is enable by using the outgoing interface which is ISP interface.

What could be a reason for this and what a troubleshooting steps can be done.

Thanks.