Sometimes when you're buying a product from your mobile devices or when you're not logged in to your account, the product doesn't appear on your Checkout page.
This problem is caused by caching.
It seems the checkout page is either cached from any cache plugin or the server-level cache.
A simple solution is to exclude the slug of your checkout pages from the settings of your respective cache plugin.
You can refer to this documentation for step-by-step instructions on how to do it.
Once you're done, it will help solve this problem.
