Not spawning mobs

So me and my friend made a mob farm on our 1 year old world and i tried the mob farm in creative to start with so i could know for sure it would work  and it did so me and my friend made it and befor this mobs havnt been spawning which has been a trouble for us such as zombifying villagers and mob farms so i have come to report this so our mob farm we spent 2 hours on works, and the mob farm did say it was bedrock and the video came out a week ago so it was in the newest update.

Thank you for reading this