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 insurance has been terminated and I am no longer eligible because of my age. I thought full time students who are 21 and under can still be placed as a dependant? Is there a chance they just didnt receive my mail? If thats so, then why didnt they state that they didnt receive the required proof of enrollment instead of terminating me for my age? I want to call them but they are closed for the weekend, so any advice from you guys so I know what I am saying when im on the phone with them would be helpful. Btw it is not grades, I have a 3.5 gpa. All those telling me to call them up is useless since I already plan to, what I need is any advice on what to say when I do call them on monday, since they are closed for the weekend.