Airtel - Not permission to recharge my own

I am having Airtel sim on which plan *** activated till */**/**** and balance of sim is Rs ***. I want to recharge again with the same plan of Rs *** for further use. When I tried to recharge the plan with my own from the sim balance it restricts me to do the same and message is coming from the Airtel stating that insufficient balance, you need Rs *** to do this recharge. I don’t know why it is happening even having sufficient balance. Talked twice with the customer care representative but they are not providing the solution for the same. I request them to lodge a complaint about the same but they denied. Now please look into the matter ASAP.