What's wrong with my lighting function in the shader?

Question

I'm struggling to find the error in my ultra-simplistic lighting function in my vertex shader:

in vec4 position;           /* Homogenized input vertex position.           */
in vec4 color;              /* Input color information.                     */
in vec3 normal;             /* Normal vector to the surface.                */

uniform mat4 ortho;         /* Orthographic matrix.         */
uniform mat4 model;         /* Modelling matrix.            */
uniform mat4 view;          /* View transformation.         */
uniform mat4 project;       /* Projection matrix.           */

out vec4 color_out;         /* Color passed to the fragment shader. */

float light()
{
    mat3 normat = transpose(inverse(mat3(view * model)));
    vec3 norm  = normalize(normat * normalize(normal));
    vec3 light = normalize(vec3(1.0f, 1.0f, 1.0f));
    return max(dot(norm, light), 0.0f);
}
void main()
{
    gl_Position = ortho * project * view * model * position;
    color_out = vec4(color.rgb * light(), color.a);
}

This function is used to illuminate the faces of a rotating icosphere. Since I've manually generated the vertices of the icosphere and I can access them at any time, I've tried to replicate the same operation in CPU code showing the result for one face, e.g.:

Normal Matrix:                 Normal vectors for face 10:
 [ 0.08     0.59    -0.81]     { 3936}[-0.58     0.61    -0.54] --> light: 0.694
 [-0.00     0.81     0.59]     { 3937}[-0.58     0.61    -0.54] --> light: 0.694
 [ 1.00    -0.04     0.06]     { 3938}[-0.58     0.61    -0.54] --> light: 0.694

Despite the normal vectors being correct or not, the light value does change with the sphere rotation. However, if I run the code (and let the shader compute the value), the resulting triangular faces appear completely dark.

This is how I copy vertex data to the buffer and how I point the attributes:

/* (...) Bind shader, and get attribute locations. */
/* (...) Create VBO. */
glBufferData(GL_ARRAY_BUFFER, vertices.size() * sizeof(Vertex), vertices.data(), GL_DYNAMIC_DRAW);
/* (...) */

glVertexAttribPointer(position_id, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), 0);
glVertexAttribPointer(color_id,    4, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void *)sizeof(glm::vec3));
glVertexAttribPointer(normal_id,   3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void *)(sizeof(glm::vec3) + sizeof(glm::vec4)));

/* For reference: vertices is a std::vector<Vertex>, where Vertex 
 * is defined as: 
 *
 * struct Vertex {
 *     glm::vec3 p; ---> Position. 
 *     glm::vec4 c; ---> Color.    
 *     glm::vec3 n; ---> Normal.   
 * };
 **/

Answer 1

Silly me, I forgot to enable the normal vertex attribute... This is the line I was missing:

glEnableVertexAttribArray(normal_id);

Where the normal_id had previously been bound with glGetAttribLocation.