This is the worst one, so let us be precise about it.
Your customer pays. The money arrives at Mollie. But on your website, nothing happens: no package, no confirmation, no email. From your customer’s point of view, they have paid you and got nothing.
What is going wrong
When a payment completes, Mollie calls your website to say so. That call is the webhook.
Your website then activates the package, generates the invoice and sends the email.
If Mollie cannot reach your website, none of that happens. The payment is perfectly fine at Mollie’s end. Your website simply never heard about it.
Confirm it in ten seconds
Go to Mollie Payments, then the API keys tab.
Scroll down to Webhook URL, and click Test webhook.
If that fails, you have found your problem.
You can also check the System tab, in the Health section, which will warn you if the webhook is not configured.
What blocks a webhook
Something on your website is refusing Mollie’s call. It is nearly always one of these five.
A security plugin. Wordfence, iThemes Security and their relatives block unfamiliar incoming requests. Add an exception for Mollie, or temporarily switch the plugin off to confirm that is the cause.
A firewall at your host. Some hosts block requests they do not recognise. Ask them to allow incoming requests from Mollie.
Maintenance mode or a “coming soon” page. These hide your entire website from everyone, including Mollie. Switch it off.
Password protection. Some staging sites sit behind an HTTP password. Mollie does not know the password.
A site that is not publicly reachable. If you are working on a local machine or an internal network, Mollie cannot reach you at all. This is normal, and it is one reason to test on a real, public staging site.
Fixing the payment that already got stuck
The plugin does not just rely on the webhook. It also checks in with Mollie every hour and catches anything that was missed.
So a stuck payment usually sorts itself out within the hour, without you doing anything.
If you cannot wait, open the Transactions tab, find the payment, and refresh its status manually.
But do fix the underlying webhook problem, because otherwise every single payment will be an hour late, and your customers will not enjoy that.