Encryption Paused: connect power adapter to resume encryption

Why cant I resume the encryption. I connect the power and nothing happens. I retart with power added and I get a "Calculating time remaining" then right back to the title message. I just bought this Macbook Air and already this issue has happened and its drawing quite a bit of battery power. Also Apps using significant energy shows "Reverting from Filevault"...this does not look good on Mac