I agree that the declaration of the increment was made in April 2014. However, the wage increment was given with retrospective effect, i.e., January 2014, during which period you were in the service of your employer. In the absence of any terms in written form disqualifying you from receiving such an increment, you are entitled to these benefits for the period you were in service until your retirement.
You may please draft a letter to your employer claiming these benefits, including the difference in your retirement benefits, and send it to them by RPAD. They will definitely respond to you in writing. If they fail to respond, you may address this issue through a proper legal forum. Since you are now retired, you can approach this issue as an interesting one.
Thanks and regards,
V Sridhar