People may not be signing up for your casino links via adult pop-under traffic for several reasons. First, the traffic may not be highly targeted, meaning visitors are not interested in casino offers. Second, the pop-under format might be seen as intrusive or annoying, leading to high bounce rates. The design and messaging of your landing page could also be unappealing or not optimized for conversions. If your offer doesn't align with the interests of the adult audience, they are less likely to sign up. Lastly, you may need to test different ad creatives and refine your targeting to attract more relevant visitors.