Why was my health insurance terminated when I am 20 and fulltime college student?

I am a dependant on my mother's health insurance from her employer. Recently I was sent a mail requesting a proof of enrollment to show that I am indeed a full-time student. I put in a proof of enrollment that I am taking the 12 units in college, plus I also sent a copy of my schedule for this semester. Today I get a letter saying that my [...]