Keep/lose track of somebody/something
Definition 1 of 2
Pro Tip 1/3Stay Informed
Use when emphasizing the importance of being updated or informed about someone or something. 
With our app, you can keep track of your daily fitness activities.
Pro Tip 2/3Active Effort
Implies that a person is actively working to stay aware of certain information or someone's activities. 
You must keep track of the project's deadlines to ensure timely completion.
Pro Tip 3/3Potential Difficulty
Suggests there may be challenges to maintaining awareness or oversight. 
I find it hard to keep track of all the characters in this TV series.