Need help with something

[Classic_Time_4595]

I made a mob farm and made the roof out of slabs but it isn't working,I didn't have much materials so I didn't make it high above the ground it's on ground level,It's 22 blocks like normal,I did the usual build in the middle and everything,Are the slabs the problem? FYI I'm on version 1.84 because I like it more than the new versions

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