When i use a spawn egg on a spawner it doesn't change the spawning mob inside and nothing spawns. I get the achievment but nothing happens, i tried it in creative and same issue there whereas in vanilla it works fine. Maybe I'm doing something wrong I'm not sure, please let me know thank you!

Are you using a modpack?

Yes I am playing on valhelsia III

valhelsia blacklists a significant amount of spawn eggs from spawners.

Ok. Thanks for the quick reply really apreatiate it!