Clip Space View Frustrum Culling Not Working Properly

Question

I don't understand why this happening. I included the code that I am using to check if a vertex is inside the view frustum.

bool isPointInsideViewFrustrum(f4x4 mvp, float4 vertex)
{
    float4 clipSpace = mul(mvp, vertex);

    return ((clipSpace.x > -clipSpace.w) &&
       (clipSpace.x < clipSpace.w) &&
       (clipSpace.y > -clipSpace.w) &&
       (clipSpace.y < clipSpace.w) &&
       (clipSpace.z > -clipSpace.w) &&
       (clipSpace.z < clipSpace.w));
}