Hello there 
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I am the co-founder of AfterSell, one of the leading post-purchase upsell apps on Shopify. Happy to shed some light on why this is occurring!
Unfortunately this is a limitation of the Shopify Post-Purchase Extension. As mentioned on Shopify’s dev documentation, it is not possible for an app to “add a subscription to an order with an existing subscription”.
So if the original order contains a subscription product, and the upsell product you have set up is also a subscription product, the offer will not be shown. This limitation would be present for all the major post-purchase upsell app solutions (since all of the apps use the Shopify Post-Purchase Extension). That is the reason why the offer is not appearing for you despite all the different apps you have tried.
That being said, if the original order does not contain a subscription product, you will be able to upsell a subscription. Below is an example of what a post-purchase subscription upsell would look like on our app AfterSell. AfterSell is compatible with a variety of subscription apps including Recharge, Skio, Smartrr, Prive, Appstle, Loop, and nearly every other subscription app on the Shopify App Store.
This is likely not the outcome you were looking for, and it is unfortunate. But hopefully in the future there are updates Shopify makes to their extension that address this limitation.