Admin has to relogin

Dears, I'm Admin for Sysaid, and some of mij colleagues have to re-login evertime they open the Sysaid Admin portal. Some of the admins only have to re-login a few times a week. I never have to re-login. What is the cause of this change in login? We already tried deleting cache, cookies, history but there was no change. Thanks for your help, kind regards Anthony