CARTS/CREATE webhook keep failing

Shopify Partner
46 0 3

My app subscribed to carts/create webhook. But recently keep getting  the following error email  saying "Your webhook for carts/create  is failing to return a successful response.
This webhook has been attempted 4 times. If your webhook continues to fail, it will be removed and your application will not receive any more notifications.". I am already using the best practice to handle the webhook. I am giving back 200 response as soon as the webhook arrives then only start to process  and  I have already registered several other webhooks too. But the problems happen for this particular one topic. My app runs on google app engine.  Any help or insight appreciated. The webhook is responsive. We are continuously monitoring it.