Monster sheets in DnD5e and ADnD2e only copying and not being individual

Sometimes (not all the time) when I set up a combat between my players and some monsters, in both when I come to mark off what HP the monsters have left, the change will affect them all and not just the specific monster the character hit. What causes this and how can I make it just affect the one monster being attacked?

Hi Lexin, This happens when the tokens are improperly set up. Make sure that non-unique monsters do not have HP set in the token settings. Make sure it is set to "None" instead, then type the HP into the token. In the image below the green box is correctly set up for non-unique monsters. The red box is incorrectly set up for non-unique monsters (correct for PCs and unique monsters/NPCs).  

Brilliant, thank you!