I have had a couple people ask about not being able to tag members lately. This is often due to either a voluntary(to take a break) or involuntary suspension.
Regular members are not able to tag someone when they are on suspension. At least thats how the software is supposed to work. I have seen some tagged while suspended. It may be dependent on whether their profile is public or private as well as to whether can tag on suspension. I’m not 100% sure on that.
Anyway, point being, this is often the reason that users cannot be tagged.