Council Tax Benefit

I am on a low income at the moment which is (£130 a week) and also I get child tax credit and working tax credit. My son is a full time student and he's 19. I've just been told that I'm no longer entitled to council tax benefit(even though I've let them know that my son is a full time student) could anyone tell me why I'm not entitled because my income has remained the same.

Thanks