What Is Problem?
If you have a custom checkout page and a buyer needs to login, WooCommerce will redirect them to the global checkout page instead of your custom checkout page where they were intending to buy.
Install the free Previous Page Redirect For WooCommerce plugin.
This will remember the last (checkout) page the buyer was on and redirect them back there instead of the global checkout page.
Alternate Code Solution
If you prefer, you can add this function to your child theme functions.php (or snippets plugin) to achieve similar result (don’t use with plugin!)