How to make a WebCamTexture 'run'?

I'm trying to use a WebCamTexture object to take a picture when a button is pressed, but I'm stuck with the following error:

 Cannot get pixels when webcam is not running
 UnityEngine.WebCamTexture:GetPixels()

Here's my code (note that it's only the code regarding the problem):

     private WebCamTexture _cam;
     private Texture2D _picture;
 
     void Start() {
         _cam = new WebCamTexture();
 
         if (WebCamTexture.devices.Length > 1) {
             _cam = new WebCamTexture(WebCamTexture.devices[1].name);
             Debug.Log("Set WebCamTexture to device 2/" + WebCamTexture.devices.Length);
         }
         else if (WebCamTexture.devices.Length > 0) { // This returns true in my case
             _cam = new WebCamTexture(WebCamTexture.devices[0].name);
             Debug.Log("Set WebCamTexture to 1/" + WebCamTexture.devices.Length);
         }
 
         _cam.Play();
     }
 
     private bool UpdatePicture() {
         if (!_cam.isPlaying) {
             _cam.Play();
         }
 
         if (_picture == null)
             _picture = new Texture2D(_cam.width, _cam.height, TextureFormat.RGB24, false);
 
         if (_cam.didUpdateThisFrame) {
             _picture.SetPixels(_cam.GetPixels());
 
             _picture.Apply();
             return true;
         }
         return false;
     }

The problem is at the if(_cam.didUpdateThisFrame()), this is always false and when I remove the if statement the above error occurs. Does anybody know why this may happen or how to solve this? Thanks.