A person dusts his bed, lies on his right side and recites Aayat-ul Kursi with Wudu before sleeping at night, yet he occasionally sees scary dreams. The promise of Allaah is true, so that person may be doing or thinking something wrong or there is another problem.
Can you please shed more light on this for him and the rest of the people? What is his remedy so that he may not have any more scary dreams when he sleeps at night?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

You have to know, dear brother, that the legislated mention of Allaah that one says before going to sleep is like a weapon in the hands of a soldier. However, for this weapon to be efficient when facing the enemy, it has to be carried by strong arms. So the mention of Allaah like reading verse number 255 from chapter 2, and sleeping in a state of ablution and other mention that is reported in the Sunnah are in themselves a Ruqyah from the devils and their whispers. Nonetheless, this Ruqyah must be sincerely devoted to Allaah seeking his refuge while pondering on the words that are uttered and being attentive whilst uttering them.

Uttering the mention of Allaah without the attentiveness of the heart and while pondering about the matters of this world, may not bring its effect, and thus the deficiency is in the person. It is reported in a narration that: "Allaah does not accept a supplication from a person whose heart is not attentive while making the supplication." [At-Tirmithi] 

Allaah knows best.