Does sit mean the same for you and your dog??

Dogs can be pretty bad at generalising the meaning of things
You might think you have trained your dog really well to sit but what exactly does your dog think you mean.

You like to think that 'sit' means dog still with its bum on the floor and legs straight
Its not as easy as that

Your dog might have learnt that when you are in the sitting room, with a treat in your hand, lifting the treat above its nose, when its all quiet and saying 'sit' means the dog goes from standing and puts its bum on the ground until it is told 'good boy' and given the treat then it can do what it wants

Dogs can be as specific as that

So decide EXACTLY what you want your command to mean
I want sit to mean go into the sitting position from standing, lying down or upside down. Do it from infront of me, to the side, behind or at a distance. Do it in the sitting room, kitchen, hall, outside, wherever. Do it when I say and/or give the hand signal, do it no matter what distractions, do it whether there is a treat there or not, and hold the position until I tell you to do something else.

So I have to train for ALL of these things - train in different locations with dog in different positions with different distractions.

So if your dog dosent do what you want ask yourself have you actually trained it how to do that SPECIFIC thing?