If your dog is collar trained, I would follow a HERE or COME command with a electronic stimulus on a higher than normal level. If this is the only problem the dog has, then it will quickly learn that not completley retrieving will result in a correction.

If you dont like the collar conditioning, you could use a long lead and give it a hard jerk along with your come or here command. The dog will quickly remember who the one in charge is. Not fully retrieving is a serious problem in my opinion - I think that this can be corrected quickly.