Transactions can be declined for many reasons. The best way to find out why a transaction was declined is by going to Sell > Transactions and then viewing the transaction. It will detail out the errors as it pertains to your payment processor (see their site for more details).
You may also have strict settings in place which we recommend you consider disabling to avoid transactions being declined.
Login to your payment processor and look for AVS and CVV settings. Make sure you review those settings to ensure they are not blocking correct payments.
If you are billing subscriptions each month, you will need to remove your CVV settings that decline transactions without a CVV as the CVV is not supplied on recurring billing.