Why would a student's number of THINK Points decrease?

Student THINK Points will increase as they are working through a lesson. The counter they see while working on the lesson is specific to the points earned in that lesson only.  When they begin a new lesson, that counter will start again at zero.  Here's what students see as they work through a lesson:

 

The student can click on "Imagine Math" at any time to return to their Home page.  From there, they will see their THINK Point balance.  And below, they can view their THINK Points This Year.  Here's what they see: 

Please see article for details about viewing Transaction History: How do Students Track THINK Points