If it’s a card you should be targeting to an enemy, I noticed the issue myself. I somehow made the hitbox kinda small. Try aiming it towards the center of the enemy’s picture. Some cards do not need a specific target (like the Continue and Guard cards) and you can drop them anywhere in the top part of the screen. Also take a look at the mana costo of the card and your available mana in the top left. I’m aware it’s kinda bothersome to have to double check, I’ll definitely be highlighting the playable cards in future. I hope you’re still having fun with the game :)