You can not fight for what is in the ctc
Ctc is cot to company, not what is payable to you.
You need to go by what the appointment letter and the salary structure says. Since it says you will get it after 12 months of work, they are quiet in their rights to deny it.
Secondly, it is performance linked incentive.
So itmismwithin their rights again to deny it saying the performance was not up to the mark.
You do not have anything to,fight that.
It's best you gap forget it and move on.
Probably in future, you will ensure that the company clearly states what u will get for part year working or clearly define the terms ofvPLI in a manner that has a quantative basis, not a vague qualitative base