Unfortunately there is no way you can force your affiliates to be active, everyone as a mind of is own. They are independent and they have to make they own decision. The only thing you can do is to motivate them by sending e-mails two times per week, or make that an offer that if they log in everyday and place a standing order you will reword them with a gift certificate or 10 or 20 TC in the hand of each month.
