There can be four reasons for this:
The customer is paying via PayPal and the payment processor is set to request manual payment (instead of automatic).
You have a payment processor that doesn't support recurring/subscription charges (with Nets you need to have a separate agreement with them).
You haven't told Simplero that your payment processor supports recurring charges. You will need to check and set up your payment processors.
Select Settings from your Simplero Dashboard
Select the Payment processors tab
Make sure to enable Quickpay as one of your payment processors
The charge is declined because the card is expired, there are not enough funds, the card is revoked, or the issuing bank is having a bad hair day (LOL). If this is the case, you'll see the message we get on the invoice, on the purchase, and in more details under Transactions. What you see is all we know.