If you are using "Simple 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 general settings
Step 2: Open the settings of "Simple Cache" under settings tab.
Step 3: See the dropdown right side of this simple cache option page "Enable Advanced Mode"
Step 4: Change the option to "Yes" to have advance mode settings.
Step 5: Locate for the option "Exception URL(s)" on left side.
Step 6: To exclude “checkouts” pages enter your noted checkout page slug (from step 1) as shown in screenshot: /checkouts/*
Step 7: Scoll down the page and click on "Save Changes" button to update the settings.
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