I agree that declaration of increment was made in April 2014. However, the wage increment was given with retrospective effect i.e January 2014, in which period you were in service of your employer. In the absence of any terms in the written form disqualifying you in getting such increment, you are entitled to such benefits for the period you were in service till your retirement. You may please draw a letter to your employer claiming such benefits inclusive of difference in your retiral benefits and send it to them by RPAD. They will definitely respond to you in writing. If they fail to respond, you may approach this issue thro' a proper logal forum. Since you being the retired person, you can approach this issue as an interesting one.
Thanks & regards,
V Sridhar