why a session becomes a head blocker

Question

I am troubleshooting a problem that seems to be random

The problem is sometimes an application (dynamics SL) is freezing

When the application is freezing , I notice there is a blocked by in SQL activity monitor and a head blocker

in my limited understanding , head blocker means a session is currently running and is locking a resource and that resource is also needed by another session. so all sessions that need access to that resource cannot continue thus the "blocked by" that I see in activity monitor

my question is in five cases that I have seen is that the head blocker task status is actually blank. usually a task status is "running" or "suspended" now how can a session that is not running becomes the head blocker?

I am thinking my understanding of head blocker maybe wrong. but please enlightened me ELI5 style :)

each of the 5 cases. I just kill the process of the head blocker. then everyone is not freezing anymore.

I am hoping to identify the source of this head blocker and come up with a solution

When I click the details of the head blocker. all I see is

select @@identity

please advise?

thank you