"Rollback Partial Payment" is greyed out ("Rollback supplier payment")

Hello, 

I have a question about the "Rollback Partial Payment". In the "Rollback supplier payment" screen, when the payment is of automatic type, sometimes the rollback partial payment is not greyed out and sometimes it is greyed out and I can't understand why. Could you please help me with this? 

Thank you very much.
Lina