There can be several reasons for delay or absence of results that are mentioned by verses in scriptures.
For example, one of the key reasons can be lack of faith. It can be safely inferred from the following words of Krishna
O son of Pritha, whatever acts of sacrifice, charity, or penance are
done without faith, are termed as “Asat.” They are useless both in
this world and the next.
[28, 17, Bhagavad Gita]
If sadhaka lacks faith in either the verses on scriptures, guru, god, etc., then there are higher chances that the particular sadhana may become useless or results may get delayed.
An excellent story for this is present in Sri Parasara Samhitha.
A sage named Puskara initiated two persons: Dhwajadatta and Gala to the Hanuman mantra. Dhwajadatta was a Brahmin and has mastery over Vedas and several other scriptures. Gala was a kirata and committed many sins. Due to the differences in faith towards the guru pushkara, results also differed. Gala achieved results by reciting just 108 times but Dhwajadatta didn't attain results even after so many times recitations of the Hanuman mantra.
Please read the following words of Pushkara saying the reason for the delay of results in the case of Dhwajadatta. You can read the verses from 27 onwards
Having decided that Puskara said so to Dhwajadatta, `Oh! Brahmin!
You do not have faith in the Guru. This lead to delay in achieving the result of the mantra, even though you recited it as stipulated.
In case of a mantra, a holy water body, a Brahmin, God, an astrologer,
a physician, a Guru; one gets result in proportion to one's
attitude. Oh! Good Brahmin! You have faith in the mantra and in the
God. But you have no faith in and devotion towards the Guru that
taught you the mantra.
[28-30, 12: The Story Devotion to the Guru, Sri Parasara Samhitha]
Thus, even if the verses say the results, they presume all these qualities from sadhaka, which may not be present in sadhaka, and hence the results will get delayed.
Note: After the discussion with OP, which you can read from the comments, I think the OP is asking for the reason for the delay in results that are mentioned by stotras. And the need for repetition. It is not restricted to the particular verses given by OP and hence I answered in a general sense.