Format if the age gap is less than 9 months

[Marcia]

Hi. The sheet has 3 fields (Family No, Member and Bdate). In the birthdate, I pasted the ff formatting formula:
=AND($a$2:$a$10,$a2,$b$2:$b$10="C",$C3-$C2<9)


where: Col A = Family No, Col B = Member, Col C = Bdate)
The birthdate of "C"s with less than 9 months gap from the other "C"s within the family should be formatted. Kindly point out what is missing in the formula.
Thank you.