NTA requires some fields to be present in flows. You can see this article for reference. Check this in WireShark or post a screenshot from WireShark showing NetFlow v9 template. If you confirm all required fields are present and still no flows are being processed I suggest to open support ticket.
our new NTA 4.1.1 beta has also some improvements for ASA devices and templates. once you check the required field let us know back.