Vee.
You nailed it with that pic!!!!! lol.
All children grow and do things at different times, and while it can be frustrating to parents and fully understandable....It happens when it happens.
You can look up on line cases and studies and even when parents share info.
But!
At the end of the day, it really is going to be up to the child, to get it done.
You can help by training each day, but I found if the little one is not ready!
Your out of luck.
I have seen parents stress over it, and the child will pick up that stress, and it can take even longer.
By making it fun, most children will pick up on it.
Patience is all you can do with this issue, and LOTS of LOVE.
That can overcome fears and worries little people have. :)