Why am I so bad at “Classic Zombies”?

I bought BO2 and BO3 with all DLC and have been grinding BO3 zombies mainly, this is my first time with the OG zombies in these games and I am so bad at it for some reason. In BO2 zombies the highest round I have gotten to is like 7, and in BO3 my highest round is 17ish. But in BO6 and Cold War I was easily able to get to the rounds of 30-40. Am I playing wrong or am I missing something? I do pack a punch fairly easy in BO3 but I haven’t in BO2 simply because of how bad I am in it.