I remember feeling so frustrated when I couldn't get Havana to lie down on command in Puppy class. Or in Puppy 2 for that matter. For crying out loud, I'm an instructor and I can't get my dog to lie down without a lure?! However, with life being as busy as it is, I didn't devote a lot of time to downs when I wasn't in class. One day I made the concerted effort to teach Havana down without the lure and boom! She understood! What on Earth happened?
I suspect it was a combination of Havana maturing and something called "latent learning". Basically, Havana learned down but not exactly how to use it in every day life. Then one day the connections were made and she understood. And a bonus for me, not only did she understand down, but she will do it from a nice distance.
What I think helped is that I started requiring a down in more high value situations. If she wanted to get out of her crate she had to down, if she wanted a treat or to come in from outside or before I'd throw a ball for her she had to lie down. The reward was great and this helped her to process the idea from a latent state to a relevant and current state.
BabyBug has done the same thing to me numerous times. In particular I remember saying to my Mom after Bug's 18 month check up that I didn't think Bug's speech was progressing as quickly as it should. The doctor wasn't concerned and I wasn't extremely worried, but it was a thought in the back of my head. Mom recently reminded me of that conversation after Bug had finally taken a breath after talking for about two minutes straight.
Like Havana, I used Bug's interests to help develop her skills. Bug is fascinated by butterflies, dogs, and vehicles so when she would see those things I would say things like "The butterfly is flying" or "Did you see the truck? Can you say 'truck'?" Many times she would just look at me and then go back to watching whatever without acknowledging what I said. Then a few days or weeks later she would belt out "Tluck! Biiiiiiiig tluck!" as soon as she saw a truck without any coaching from us.
As Havana has gotten better with downs I've been able to use them in more and more challenging situations. She was a brat at the dog park one day and I called to her to lie down and she dropped. She would down and stay for longer and longer during meal times. Havana has now progressed to the point where I can usually get a down out of her in high arousal situations like working sheep or in flyball.
Bug has been able to use her words in more challenging situations, too. Now instead of just a "big tluck" it's a "Big white tluck drive by Nana's house" or most recently "I do not watch to watch this TV". She won't be going on the public speaking circuit anytime soon, but she definitely is not speech delayed, either.
Patience may not be one of my strong points, but I'm learning to wait it out and find ways to support emerging skills without worrying that they'll never happen. Of course I'm aware of approximate milestones and know to consult a professional if there seems to be a problem, but if the professionals aren't worried, I'll try not to worry as well.