Bottom slabs letting in sky light

During the day, the roof of my automatic mob farm, which is made of stone bottom slabs, would let in sky light. I fixed it by placing full stone blocks under it.  It stopped letting sky light in before I finished placing all the blocks. After I broke some of the full blocks I placed, the sky light remained 0 so idk. Unfortunately, I didn't take any pictures but I am attaching a picture of the mob farm style that I used. I just wanted to post something because I couldn't find anyone with the same issue on java.