Playing a game on a medium world. I researched Armor (Light Plate) and when finished I attempted to update my light calvery units that were outfitted in upgradable leather armor. Clicked on the Upgrade Armor button for the unit, the metal and gold were deducted from my stores but the armor didn't upgrade. I did this for several units without success.

Earlier in the game I did successfully upgrade from no armor to leather and also was able to upgrade clubs to war maces. It appears this might only be with the plate armor option.