In Minecraft Bedrock, I built 2 mob spawners. One the drops 20-something blocks underground (I don't remember how many, but I followed a tutorial) and another above ground. The above ground one is bright inside even though there are no holes. I made both of cobblestone. I had the same issue with the first one I built a while back. The underground one is completely dark inside, I went and lit up all nearby caves and still no spawns. I have gone to the other side of the map as well so I know I'm not too close.
The reason is because mobs only spawn from 148 blocks away from caves, if it is closer, the mobs will spawn in the caves instead of the mob farm. There's a solution to this. You can build the farmer on the ground but only if you lit up all the caves and dark spots in a 148 blocks radius. You can do one out of the two, either build it high up or build it on the ground and lit the caves in a 148 blocks radius, it's up to you. Wish you good luck!