Make sure you aren't holding the treat too high as that can make the dog want to jump up. Also, some dogs want to use their paws to get the treat out of your hands. Wait for all 4 paws to be down!
Notice I haven't introduced my word SIT yet. I only use the word once the dog is perfect with the hand signal. Remember, dogs don't speak English, they use body language.
Keep and eye out for more dog training!