Once the customer has paid, the sold item moves into the "Waiting for Postage" section of the Seller Hub.
There is no reason to wait until the money has hit your bank account - you need to send it within your handling time.
If you do not send it when expected, you will definitely start getting negatives and hits to your account metrics.
Keep some reserve funds in your account to use for postage until the rolling payments occur.
As to why funds are being declined, there is a thread which mentions this - appeara to be an Ebay/Adyen issue for some.
