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You might have futa turned off go to sanctum and turn it on

Or it might be connected to Edith.

I think you get to learn Cock Growth spell after Rinny uses it on Edith

Or it might just be me completing these Quest before even properly talking to Rinny.

You have to finish the Manor quest to open all the places in Transylvania, and then make sure your Mystic skill is 7+ before asking Rinny to teach you magic. The skills she teaches you in the order I've always got them is Drain Kiss > Electric Shock > Cock Growth. If it's still not working, it might be connected to the TF fetishes, which I think is what toggles dick/tit growth, so make sure you've got the Cock TF one on and try again

As mentioned, you need cock tf and futa kinks enabled!