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 URL 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 page slug is "offer". If you had changed the offer page slug for any reason, get the slug from FunnelKit > Settings > General > Permalink.