If the orders are not completing automatically, one of the major reasons could be misconfiguration in WooCommerce or the payment processor you are using. The eCommerce plugin such as WooCommerce, Easy Digital Downloads, Paid Memberships Pro, Restrict Content Pro expects a response from the payment processor such as PayPal, Stripe, 2Checkout etc.
If the plugin is not receiving the expected response that the payment is completed, then the order status will not be updated.
Please consult your payment service provider if they support the eCommerce plugin you are using.
In the case of online bank transfer, the same process is also expected. If the bank does not provide a response after completing the payment back to your site, then the order will not be marked complete automatically.