King Mechagon did not drop Progression Sprocket

You will only receive a Progression Sprocket if you have first learned the Perfectly Timed Differential (Rank 2 of Vision of Perfection).

Please note that normal and hard mode Mechagon share the same lockout. If you kill King Mechagon in normal mode and receive one Progression Sprocket, killing him in hard mode during the same lockout will not reward additional sprockets. 

You can have a maximum of 4 sprockets at a time. If you already have 4, you will not receive another one. Please make sure you don't have any in your bank.