A lot of the problem is that students encountering different dynamic levels for the first time don’t have the technique to make different dynamic levels. They usually start by playing anything that is marked soft(p, pp, mf) as soft as possible, and anything marked loud (f, ff) as loud as possible. Even more difficult is to make a gradual crescendo or decrescendo. They will make terrace dynamics, with abrupt changes in the dynamics. I think the thing to do is to start by using dynamic changes in exercises and etudes. The student has to learn how to make the dynamic change, and only then can they start applying them to a musical context.
I don’t know if other’s may view this thesame, but I have noticed that when a begginer harpist wishes to play paino or pianissimo, they pluck the string giving it an initial very small amplitude, whilst when playing forte or fortissimo they almost break the string out of place with the initial amplitude.
I think of it as intention and dynamic energy. You are bringing up energy from your center, and as it moves through your back and shoulders and down through your arms, you lean into the harp and channel that force into your very fingertip, so it takes very little tension, just a little pressure. It must be like how when a woman stands on a stiletto heel, the force transmitted through the tiny tip of it is unbelievably strong.