If your internal root domain name and external domain name are the same then the clients could potentially get confused and try to go to an internal resource when they really should be going to an external resource. You may need to implement SPLIT DNS to get the internal hosts to get to the right website. The working internal clients may have an entry in their hosts file or some other DNS resolution server. Here’s some good info for Split DNS on this ISA article. You can also put wireshark on the working hosts and non-working hosts and compare the results when requesting the content of the website of interest.