There are several scenarios that could lead to one or more payments being delayed. Here's a list of possibilities.
1) Affiliate could be on 'New Account Probation' or on 'Administrative Probation'
2) The affiliate could be missing a PayPal email, and thus have to be paid manually
3) The Affiliate could have a PayPal email and be approved for instant, but a JV be missing a PayPal email. In that case, the Affiliate is actually paid instantly, but the transaction still shows up as needing to be released because of the JV payment.
4) The Affiliate could be approved for instant payments at this point in time, but was not approved for instant payments when the transaction occurred. This also works in reverse.
5) The Affiliate could be approved for instant payments on some products in a sales funnel but not for all products in the sales funnel
6) The payment was made through Authorize.net