I looked at your account and everything seems fine. I noticed that two of your four students had been ‘archived’. This means that the tests assigned to those students wouldn’t appear in the ‘Student Activity’ tab. Perhaps this is the cause of the confusion? If you want to see the activity for those students then just go to the ‘My Students’ tab and tick ‘Show archived Students’. Then select the archived students. Now their activity will also show in the ‘Student Activity’ tab.
Hope this makes sense