Retriever Specialists Tips:
Momentum
Momentum is expressed desire and natural drive to act. In retriever training it is also action taken under properly used conditioned compulsion.
In training your retriever, momentum should be both a combination of action under compulsion, and an expression of your dog's desire and natural drive.
Lack of momentum or a noticed loss of momentum in your dog's work is a big red flag - notice it and take action to re-establish momentum immediately.
If, in your retriever training, you compel your dog in any fashion and his momentum on that behavior slows initially but doesn't recover AND IMPROVE in a few repetitions, you must stop and figure out why.
Accept that responsibility.
Don't wait until loss or lack of momentum is a built in reflexive part of your dog's trained behaviors.
Enthusiasm, or a total lack of it are a reflection of your training.