Facebook quick filter
Mouse over the sales column and compare new vs. recurring values.
If the recurring value is significantly higher than the recurring value, this question is relevant and look at the actions.
You are finding a small amount of customers, but they are sticking and not churning from their membership/recurring product purchases. This indicates a high quality customer. You can live with a lower ROI, as long as it is trending positive or staying positive, because the customer LTV for this ad should continue to climb.