Why does my mom love her granddaughter more than me? What can I do?

My niece is more important to my mom than me. She wont buy me things that i need because we 'dont have the money', then goes and spends her whole paycheck on her. i love my niece. and i understand the whole grandparent-child bond, but its making me feel horrible. ive tried talking to her, writing a note, everything. what can i do to get through to her.