If you are using the "Hyper Cache" plugin, your offer page must be out of cache to work properly.
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 "Hyper Cache" under the settings tab.
Step 3: Jump on the “Bypasses” tab.
Step 4: Scroll down the Bypasses page and locate for "Do not cache pages with URIs starting with" option
Step 5: Enable the checkbox option to use this setting
Step 6: To exclude “offer” pages enter your noted offer page slug (from step 1) as shown in screenshot: /offer/
Step 7: Scroll down the page and click on the "Save" button to update the settings.
By default, the upsell pages slug is "offer". If you had changed the offer pages slug for any reason get the slug from FunnelKit (formerly WooFunnels) > Settings > General > Permalinks