That's not unusual, especially for a dog that is essentially a teenager at 1 year old. Also, repeating a cue multiple times has likely already poisoned the word. She now thinks that the cue is "sit, sit, sit, sit, sit" and not just "sit". Additionally, young dogs push boundaries and have been known to "forget" cues and stare at you like you've never given them an ounce of training. Give the cue once, and if they don't perform the behavior, take a step back and lure them into the behavior as if you were training it for the first time.
There is nothing to be done other than keep practicing. Practice in a boring spot, like your living room. Then practice in a more exciting place, like your yard. Then up the criteria and practice in a public park. Dogs don't generalize well, and they don't always know that "sit" means the same thing in your living room as it does in the park. Keep training sessions short and fun.
Once your dog has the cues learned when she knows you have treats and will give one for ever command, start giving treats intermittently. I would recommend you only start doing this once the dog has learned the joy of working with you, though, not when they're a young dog that likes to do their own thing. This generally makes the dogs work harder for that treat, because now they never know when it's coming!
