IMO the problem you are having began when you started your training. You must train the dog to follow the beam, this can be easily archived.
You can tie in this training with the sit, stay, retrieve command, once you have your dog retrieving 100% to hand during the day, you can then begin to work your dog on the retrieve during darkness.
Here’s how I go about training my dogs.
As mentioned earlier the dog must be 100% on the day time retrieve, once I have achieved this I then bring the dog out when its dark, the back garden is fine for this training. Command the dog to sit, lob t