When viewing a course's logs, the ability to view individual enrolled users activity is no longer an option if about 1,000 or more participants are enrolled in the course. In my test, the drop-down menu option disappeared between 900 and 1,000 enrolled users.

To replicate:

1. Create a course and enroll a few students;
2. Navigate to Course administration > Reports > Logs;
3. Note that "All participants" appears as a drop-down menu option.
4. Enroll about 1,000 or more additional students;
5. Navigate to Course administration > Reports > Logs;
6. Note that "All participants" no longer appears as a drop-down menu option.

Workaround: Download all desired logs and filter on desired user using MS Excel or similar program.