If you follow the Webhook Trigger pattern for your own custom connector or use a connector build with this pattern, https://docs.microsoft.com/en-us/azure/logic-apps/logic-apps-create-api-app#webhook-triggers, errors happening in registering the WebHook will not appear during the Azure Logic App Designer use and will be easily missed. In the designer you will still be able to save with no validation error as the Logic App definition checks out:

However to make sure the WebHook trigger registration actually worked, you need to inspect the trigger history (expect about a minute delay for the call(s) to complete and show up in the log):

In this example I caused registration failure through bad user input, then corrected it:

You can identify the registration from its inputs, note the URL path suffix (webhooktrigger/subscribe) of the subscribe operation:

The outputs will contain the error details from which you can inform your troubleshooting: