Why do my customers have to manually go in and pay the next installment/renewal of their subscription?

There can be three reasons for this:

1. The customer is paying via PayPal, and the payment processor is set to request manual payment (instead of automatic). Bear in mind that automatic PayPal payments are only allowed in certain Simplero plans. 

2. You have a payment processor that doesn't support recurring/subscription charges (with Nets, you need to have a separate agreement with them).

3. The charge is declined because the card has 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.