Most [Updater names] coming in as null

I'm trying to pull a report on how many times in a given period each agent has updated a custom field. When I build the report in Support:Updates (and use a custom attribute to tell it that I only want it to count updates for when a custom checkbox went from "0" to "1"), it gives me some of the correct tickets for each agent, but not all of them. The vast majority end up in "null" without an updater name attached. How is that possible? How can I get accurate information about who clicked the checkbox? Or at least about who was assigned to the ticket when the checkbox got clicked?