I can tell you one thing, you're not ever going to understand the methods they've developed until you use them for the purpose they were developed. Retrievers. At that point, understanding to "read" the dog working where and why the program was developed and why the specific drills and stages are in what order. So, picking and choosing little ideas "might" give you something here and there but, you won't ever understand "why" if you're not retriever training with someone standing next to you saying, "remember "X" drill" this is "why" we do it....
So, my "read" is that your dogs are neither bored nor lazy. They are probably confused which is why it takes one so long to learn. Confusion and uncertainty can cause an animal to shut down, not want to work or, work very slowly because they are trying not to get into trouble. Add years of trying new stuff, mixing ideas, taking from here and there without understanding what those methods are for probably has the dog about shut down...