I imagine that the card (tga file) is well named. It that's true:

The existing unit cards are included in a pack. If this pack remains in its place, the game will look there first and your new cards will never been shown. You have three options:

- Unpack all the cards and eliminate the corresponding pack (save it in a safe place ). Then, replace the old card by the new one.

- Make a new patch.pak file. You can find a short tutorial in this thread:
https://forums.totalwar.org/vb/showthread.php?t=40313
The idea of the patch is in post 30.

- Rename the unit in export_descr_unit. It is enough to put a number at the end of the name. The only problem is that you must change the name in ALL the files of the game where this unit appears (e.g. descr_strat, all the historical battles, and so on).

Good luck.