[crane3]

I'm assuming that "kindle" is referring mostly to the ereader; don't know if there was just a "fire" tablet.

But what is the difference between the HD & HDX that makes apps incompatible?

The compatibility shouldn't be based on the screen resolution or even the OS version (sort of). Apps for the "android" tablets on Google Play just works except for some earlier version of android; the screen resolutions didn't matter unless something like Aldiko is doing bad programming.

I've installed apps via extracting from the Nexus 7 or Galaxy Tab Pro 8.4 & it worked on the Fire HDX. But Amazon claims that some apps available in the Kindle store isn't compatible with my Fire HDX 8.9 & won't allow installation or purchase even if they are the same apps that I was able to install & use on the Fire HDX.

For instance, I had emailed Mantano why their product wasn't "compatible" with the Fire HDX & got a reply that they didn't know & Amazon won't tell them why it was incompatible; yet the same product in the app store looks like it is available for the fire, hd or kindle!

What is/are the HIDDEN features that made a product incompatible with an HDX?