Raycasting does not work with imported object (mesh colliders are generated)

I have imported a surface which I modeled with ‘blender’, I imported it with mesh collider. When I want to do a ray casting on this surface to place random objects it won’t work. When I set a standard plane above this surface, the ray casting works on this plane. So I think my modeled surface isn’t catching the ray casting, but I don’t know why. Can anybody help me?

using UnityEngine;
using System.Collections;

public class RandomlyPlaceObjectsOnSurface : MonoBehaviour
{
    public GameObject[] objectsToSpawn;
    public float spawnRadius = 10.0f;
    public int numberOfObjects = 10;

    // Use this for initialization
    void Start ()
    {
        for(int i = 0; i < numberOfObjects; i++)
        {
            //What we will spawn
            GameObject objectToSpawn = objectsToSpawn[Random.Range(0,objectsToSpawn.Length)];

            Vector2 spawnPositionV2 = Random.insideUnitCircle*spawnRadius;

            Vector3 spawnPosition = new Vector3(spawnPositionV2.x,0.0f,spawnPositionV2.y);

            Vector3 transformOffsetSpawnPosition = transform.position+spawnPosition;

            RaycastHit hit;

            if (Physics.Raycast (transformOffsetSpawnPosition, Vector3.down, out hit)) {
             
                Vector3 finalSpanPosition = hit.point;
                Instantiate (objectToSpawn, finalSpanPosition, Quaternion.identity);
         
            }
        }
    }
}

This is my code.

Can you please post some screenshots of the components attached to your imported object!

It does work if I enable Convex on the meshcollider but then my FPScontroller just falls right through. Thanks in advance for helping!