Encrypted POST data not decrypted

To demonstrate SSL decryption, I created a dummy website running on a local webserver which prompts the user to login using HTTPS. I analyzed the traffic using WireShark and have successfully decrypted the SSL traffic. The cipher used is TLS_RSA_WITH_AES_256_CBC_SHA. However, the POST data (Line-based text data: application/x-www-form-urlencoded) containing the username and password remains encrypted. Any suggestions how to decrypt this data?