Using Occlusion Culling but some hidden objects are still rendered

I’m using occlusion culling and it does work on some objects but some doesn’t, as you can see in the image, the game view shows that camera cannot see the whole block ahead due to the cub that occludes it but some part of it are still rendered according to the visibility line ( green line ). Can anyone give me a solution to this ? I have been so frustrated for days trying to fix this problem.

Occlusion culling, on CPU side, is performed on AABB data (bounding boxes) and not per vertex/triangle. To understand this better find a way to visualize them.