Glad to hear it's going better. Just some things to consider, to make sure none of these factors are at play or to help others who might be reading this and having a similar problem:
IME, often when the dog's reaction time slows, it often has something to do with the owner. It could be a lot of different things though. You might need to do more "refreshers" for motivation--any dog, no matter how old, needs occasional fun and motivational training with lots of rewards or otherwise a really good reason to do obey you even when they'd rather not. I might only have a super-fun obedience session with my older dogs once or twice a month and just expect obedience the rest of the time, but they still get those sessions.
It's kind of like getting an unusual reward for your performance on a normal work project. You would have done the project anyway, but when your boss calls you in and praises you and gives you a token of appreciation (even if it's just something small like free movie tickets), you're likely to approach the next project with renewed enthusiasm. Dogs need that motivational stuff too, and maybe a little more often since they're not getting a paycheck for obedience.
It could also be that you phased out rewards too quickly. When you're establishing a behavior, you start by rewarding heavily each time, then you start doing smaller rewards but still consistent ones (food or toy rewards, that is), then you start making the food/toy reward inconsistent and gradually phase it out for the most part. Sometimes if people go through that process too quickly, the dog loses motivation and their obedience decreases.
As others have said, it could also be something in your body language or timing. Someone already mentioned saying the cue more than once, which is a common problem (which I saw you don't do too
). Other things I have seen are things like teaching the command in a firm voice, "PLATZ!" and then expecting the dog to respond to a more casual quiet "platz," without training the dog to do so. Body language can also be an issue. If you taught the dog the cue while standing close and facing her, you may have to teach her that it still means the same thing when you're sitting down, turned partially away, etc. This is a problem I ran into a lot when I first started training my dogs, so now I routinely teach my dogs to respond to the command in a variety of tones and with me in a variety of postures.
Or, it could be nothing to do with you and she simply hit an age where she took a couple of steps back. I think that's a pretty normal thing in training animals. We tend expect a logical progression, but in fact often you wind up hitting plateaus, or your dog takes a step or two back for no apparent reason and you need to build them back up, etc. The nice thing is that if you deal with it promptly and correctly (as it sounds like you are), it should just be a minor hiccup.