OWA question

I have a e2k server and some people can access it from out of the office and some can’t. All mailboxes are set up the same and the people that can’t access it out of the office only happens occasionally. They can sometimes and other times it keeps asking for their username and password over and over again without any errors. Example: My boss goes out of town w/ his laptop and checks his email from wherever. Sometimes it works, other times it doesn’t. Any ideas?