strange behaviour of server

ISPconfig3 shows that all services are running ok.

But some domains work nor some do not while all of them are hosted in the same server.

dig resolves all domains pointed to the right IPs.

Could not figure out what could be wrong?

 

"do not work" is quite a bad description of a problem
What error appears, what exactly doesn't work?

 

There are no errors in the log because all services are working alright (httpd, postifix, pdns, mysql).

dig shows the domain names resolved, when tried to access through the web browser, some domains are accessible and some domains fails with a blank page.

 

A correctly working httpd does not mean php is working correctly.
Have you checked all the web's error logs?