Ranger learned how to sit and mastered it by 12 wks. You really have to be persistent and patient!
When I taught Ranger I would say Sit and using a treat move it over his head forcing him to sit. Once his bottom hit the floor I would say “Good Boy, Sit” and give him the treat. I would walk a few feet so he would no longer be sitting and repeat. This went on for about 10-20 minutes and I would stop. I repeated the same thing the next day for about 10 minutes. After a few days he got it, but then he had to be weaned off of expecting a treat every time. Now he will sit on command with or without a treat!!
He is now 5 months and knows sit, lie down, sit up, stand, speak, and shake. We are currently working on stay and come, but with holidays coming up it might be a while
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Good Luck!!