light up dark areas 9x9 chunks around it(spawner in center)... more air blocks around wherever they should spawn.. etc. You should post pictures of the design or describe it
Use the /entc command, that will show you how many mobs there are in your surrounding, you can then use that info to determine if the area could be 'saturated' or not. (for example: if there are 230 different mobs around you then there are only 20 free slots, so you won't get a high spawn rate). (edit): also, you might want to use this section for questions next time: https://empireminecraft.com/forums/community-discussion.7/ See, we're talking in the 'forum games' section and for a moment I thought that this was a new game of some sort (I hardly play forum games) so I skipped it at first. And I'm pretty sure others might not expect questions here either Don't worry about it for now, but it might be handy to keep in mind if you have other questions at a later time.
I dunno how to make this picture bigger, but basicly you should be standing so the mob spawn blocks are from 24 to 32 blocks away from you....
I tried another design, same result Entity Report 17/250 ! I used to build this tower in survival, et and it worked really good. Finally the spawn rate...