This is because the default trigger position is set to 50% memory before the trigger and 50% memory after the trigger. Since the Logic Analyzer is triggering off the first thing it sees, it does not get a chance to fill up the first 50%. To fix this, go to the trigger setup menu and change the trigger position to 0% or check the "force main prefil" box or if this is unavailable (some models) then try sliding the "trigger position" slider as far as you can to the left.