Blank screen when looking at Activity Monitor details

I'm looking at the Activity Monitor in SQL Server 2005 and when I double click a spe开发者_开发问答cific process to view the SQL I am given a blank screen.

What does this mean?

What could be causing this?

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The running SQL is not stored for long, so perhaps the process finished executing the SQL some time ago? Look at the 'Last Batch' column, anything over fifteen minutes old is likely gone already.