If you are using "WP Fastest Cache" plugin, you may encounter some issues when visiting the checkout. This is due to aggressive page caching (which is a good thing!), but it can be easily resolved by excluding the checkout page URI from cache plugin.
You need to exclude checkout pages by following these instructions-
Step 1: Note down the slug of your checkout page from the WooFunnels general settings tab
Step 2: Open the settings of "WP Fatest Cache"
Step 3: Jump on the “Exclude” tab.
Step 4: Click on the “Add New Rule”
Step 5: Select "contain" as showing below from dropdown and enter your noted checkout page slug (from step 1)
The final setup like this ->
Step 6: Click on Save button to close this popup and exclude the Checkouts page from cache.
Here the final settings you saved
Note: By default, Checkout page's slug is "checkouts". If you had changed the checkout pages slug for any reason get the slug from WooFunnels > Settings > General > Permalinks