I find it get greyed out if you use it to sign in and then use that to sign in a different user you then try and do it again it wont allow you. So you need to log off and login again before using it.

If I am signing in as another user, I either will do it in a second browser (i.e. in Edge vs. Chrome) or in an In Private/Incognito window as that reduces the ghosting/cookies that seem to sometimes stick around when using the log in as user feature (especially if you have multiple windows open both as “you” and as “them”)

@Aschneid If you sign out and sign back in it will let you do it. This could be because you’ve already used that feature to sign in as someone else in your current session. If you need to be signed in simultaneously as different users use different browsers.

Thanks everyone, I appreciate the quick responses.