Getting sign in loop.

Hi there everyone, I'm a support specialist for my company and we're running into an issue. When my coworkers try to login to Zendesk it takes them for a sign in loop where instead of the site logging in, it redirects them to the same sign in page. It happens to some accounts more then others, and on some computers more then others. I've tried having them clear the cache, close and then reopen the browser, restart their computers, use different browsers. etc. and nothing has given solid results. I have had them clear their cache and restart the browser which sometimes gives temporary results, but then it reverts. back to the sign in loop. Any help is appreciated, thank you!