[Marileangorifurnimaluim]

As Frodo drew nearer to Mordor, the power of the ring increased. The ring manipulates existing weaknesses, feelings, desires. Frodo's greatest problem after the breaking of the company was his self-doubt and hopelessness.

Like the ring, the cry of the Nazgul also had much more power near Mordor, and cut to Frodo's sense of hopelessness. And much more power when there was something for people to fear or feel hopeless about. Remember even cheery Pippin, standing at the walls of Gondor, was effected by the Nazgul flying overhead.

In Emyn Muil, Frodo was feeling particularly helpless as well. They were lost.

Later, when Frodo was outside the gates of Minas Morgul, the mere presence of the Nazgul caused him to freeze in a state of total helplessness - they didn't have to make a sound. Then Sam had to keep Frodo from going to them. Their influence was even stronger.

You're correct that hunger, et al, had something to do with it. But the effect of the cry of the Nazgul was mostly emotional.

- Maril