Unlike its counterpart in Reality, the isolated squares have double jellies. 20 moves are usually not enough to hit these squares an additional time while also having to demolish the candy bombs within 9 moves (but moon struck may help you out to remove bombs).
When the first layer of jelly in the isolated squares is cleared with the candy, the game will not spawn another candy, making it more difficult to clear the second layer of jelly.
The jellies are worth 18,000 points. Hence, an additional 22,000 points is required to earn one star.
While the bombs at the bottom takes at most a few tries, the liquorice locks and jellies are much harder to reach. It is still possible to make a striped candy at the top rows, but colour bombs are more efficient to do the job. With careful plans, the jellies at the top would eventually be cleared.