Diamond Square algorithm height problem

I'm currently trying to implement the Diamond Square algorithm in unity. When I generate a terrain I always end up to flat areas at max height like so :

Here is my code :

     // Check if the point (x, y) exist
     private bool isInRange(int x, int y)
     {
         if(x >= 0 && x < resolution && y >= 0 && y < resolution)
             return true;
 
         else
             return false;
     }
 
     // Do the average of the square corners height
     private float sqrAvr(int x, int y, int step)
     {
         float average = 0.0f;
         float counter = 0.0f;
 
         if(isInRange(x - step, y - step))
         {
             average += heightMap[x - step, y - step];
             counter++;
         }
 
         if(isInRange(x - step, y + step))
         {
             average += heightMap[x - step, y + step];
             counter++;
         }
 
         if(isInRange(x + step, y + step))
         {
             average += heightMap[x + step, y + step];
             counter++;
         }
 
         if(isInRange(x + step, y - step))
         {
             average += heightMap[x + step, y - step];
             counter++;
         }
 
         return average / counter;
     }
 
     // Do the average of the diamond corners height
     private float diaAvr(int x, int y, int step)
     {
         float average = 0.0f;
         float counter = 0.0f;
         
         if(isInRange(x - step, y))
         {
             average += heightMap[x - step, y];
             counter++;
         }
         
         if(isInRange(x + step, y))
         {
             average += heightMap[x + step, y];
             counter++;
         }
         
         if(isInRange(x, y + step))
         {
             average += heightMap[x, y + step];
             counter++;
         }
         
         if(isInRange(x, y - step))
         {
             average += heightMap[x, y - step];
             counter++;
         }
         
         return average / counter;
     }
 
     private void generateTerrain()
     {
         // Corners initialisation
         heightMap[0, 0] = Random.Range(0, maxHeight);
         heightMap[0, resolution - 1] = Random.Range(0, maxHeight);
         heightMap[resolution - 1, 0] = Random.Range(0, maxHeight);
         heightMap[resolution - 1, resolution - 1] = Random.Range(0, maxHeight);
 
         // Generation of the rest of the map
         int step = resolution;
 
         while(step > 1)
         {
             Debug.Log(step);
 
             int halfStep = step / 2;
             float scale = step * roughness;
 
             // Square part
             for(int x = halfStep ; x < resolution ; x += step)
                 for(int y = halfStep ; y < resolution ; y += step)
                     heightMap[x, y] = sqrAvr(x, y, halfStep) + Random.value * 2 * scale - scale;
 
             // Diamond part
             for(int x = 0 ; x < resolution ; x += halfStep)
                 for(int y = (x + halfStep) % step ; y < resolution ; y += step)
                     heightMap[x, y] = diaAvr(x, y, halfStep) + Random.value * 2 * scale - scale;
 
             step /= 2;
         }
     }
 
     public void Start()
     {
         terrain = Terrain.activeTerrain;
         heightMap = new float[resolution, resolution];
         generateTerrain();
         terrain.terrainData.SetHeights(0, 0, heightMap);
     }

Hope I can find an answer here. :)