Your problem may lie in proving that your mother lived in the US for the required amount of time ... 10 years before your birth (not necessarily the last 10 years before you were born, just 10 years total), including 5 years after her 14th birthday. Digging up those 40-50 year old documents won't be easy.
And it may be hard to prove that she was a US citizen before you were born, if she doesn't have a certificate of citizenship showing she had citizenship at birth, or an old US passport issued before you were born.
PD: Jan 2003 (EB3 rest of world)
I-485 Approved: July 2007
I am a layman, not a lawyer. What I write here is not official or professional legal advice. In addition, my answers on this forum are specific to the scenarios discussed in each thread and should not be generalized to other situations.