During which time in the plot should you add elements to intensify the problem

The time in the plot where you should add elements to intensify the problem is rising action.

Explanation:

Rising action is a series of incidents that create suspense, an tension and get the interest of the reader, this includes the decisions of the characters toward the different conflicts they have to face or sometimes the beginning of a new conflict or development of a small conflict into a new one that will make the characters evolve.

the answer to the question is a