I am not sure I agree with the other members who responded. While I can't vouch for this [I am not a legal eagle], I don't think a Retention Bonus falls under the ambit of the POB Act. In fact, I don't think anything like this is covered under any specific Act/Law in India—this concept is still new to the Indian work culture.
Definition of Retention Bonus
Here's the definition of Retention Bonus, as per Wikipedia: "DEFINITION of 'Retention Bonus': A payment or reward outside of an employee's regular salary that is offered as an incentive to keep a key employee on the job during a particularly crucial business cycle, like a merger or acquisition, or during a crucial production period."
The bonus that one normally associates with the POB Act is related to past work done and the company's appreciation thereof. A Retention Bonus is essentially paid to 'retain' the employee for beyond a particular period—in your case, for more than one year. Hence, I don't think the company has erred in not paying the Retention Bonus.
This situation of yours has a similarity to threads posted in CiteHR about resigning before the Service/Bond Agreement has completed but the individual completing the full term of the period, albeit some of it during the Notice Period, and the situation/interpretation thereof. In a similar category would also fall likes of Joining Bonus, etc.
Regards,
TS