If you host your site on "SiteGround" and using "SG Optimzer" Plugin, you may encounter some issues with empty carts when visiting the checkout page. This is due to aggressive page caching (which is a good thing!), but it can be easily resolved by requesting that WP Engine setup a special cache exception for your checkout page.
You need to exclude checkout pages by following these instructions-
Step 1: Note down the slug of your checkout page from here
Step 2: Open the settings of "SG Optimizer".
Step 3: Locate for the option "Excluding URLs" on left side.
Step 4: To exclude “checkouts” pages enter your noted checkout page slug (from step 1) as shown in screenshot: checkouts/*
Step 5: Click on Exclude button to add this slug as a exclusion.
Step 6: Here is final output slug added of checkouts page to exclude from cache.
Note: By default, Checkout page's slug is "checkouts". If you had changed the checkout pages slug for any reason get the slug from Global Settings > Miscellaneous tab > Checkout Page Slug