If you are using the "W3 Total Cache" plugin, you may encounter some issues with upsell page when running the funnels. This is due to aggressive page caching (which is a good thing!), but it can be easily resolved by excluding the offer page URI from the cache plugin.
You need to exclude offer pages by following these instructions:
Step 1: Note down the slug of your offer page from the general settings
Step 2: Open the settings of “W3 Total Cache” and click on the "Page Cache" link
Step 3: Scroll down below to the section until you see the ‘Never cache the following pages’ option
Step 4: To exclude "offer" pages enter your noted offer page slug (from step 1) with special characters as shown in the screenshot: "/offer*"
Step 5: Click on "Save all settings"
By default, the upsell offer pages slug is "offer". If you had changed the offer pages slug for any reason get the slug from FunnelKit (formerly WooFunnels) > Settings > General > Permalink.