Why Shri Rama could not withdraw Brahmastra while Arjuna could do it?

 

Certain superior DIVINE weapons cannot be withdrawn once invoked.

A student learning DIVINE weapons will be guided by certain mantras, being taught by a Guru to the disciple. Once the mantras were properly learnt and recited for firmly establishing in the back of the mind of the student, they presiding deities of the divine weapons will assure the student that they would appear the moment the person remembers the mantra.

Shri Rama learnt all DIVINE weapons, alongwith their annulment mantras, from Sage Vishmitra and being a brilliant student, all mantras were established firmly in his mind within a short period. Thereafter, the presiding deities appear before him and assures him that they can be used whenever he wishes.

प्रतिगृह्य च काकुत्स्थः समालभ्य च पाणिना।

मानसा मे भविष्यध्वमिति तानभ्यचोदयत्।।1.27.25।।

Rama received the astras he felt with his hands and commanded them saying, "Live in my mind" (serve me whenever I remember you).

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Generally, most of the people, out of reverence towards Shri Rama state that Ocean was arrogant and he was afraid of Shri Rama. This is far from TRUTH.

After his appearance he states that he cannot exceed the limits fixed and nowhere he expresses his fear.

पृथिवी वायुर् आकाशम् आपो ज्योतिः च राघवः |
स्वभावे सौम्य तिष्ठन्ति शाश्वतम् मार्गम् आश्रिताः || २-२२-२५

"O, beloved Rama! Earth, wind ether, water and light remain fixed in their own nature, resorting to their eternal path."

तत् स्वभावो मम अपि एष यद् अगाधो अहम् अप्लवः |
विकारस् तु भवेद् राध एतत् ते प्रवदामि अहम् || २-२२-२६

"Therefore, I am fathomless and my nature is that it is impossible of being swum across. It becomes unnatural if I am shallow. I am telling you the following device to cross me."

न कामान् न च लोभाद् वा न भयात् पार्थिव आत्मज |
रागान्नक्राकुलजलम् स्तम्भयेयम् कथंचन || २-२२-२७

"O, prince! Neither from desire nor ambition nor fear nor from affection, I am able to solidify my waters inhabited by alligators."

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Then, Shri Rama says that the invoked DIVINE weapon cannot go waste and thus asks the ocean god where to discharge that weapon. The ocean god directs him to shoot towards the Northern parts where cruel demons were living. That discharged weapons kills those cruel demons.

तमब्रवीत्तदा रामः शृणु मे वरुणालय || २-२२-२९
अमोघोऽयम् महाबाणः कस्मिन् देशे निपात्यताम् |

Then, Rama spoke to that ocean as follows: "Listen to me. This great arrow should not go in vain. In which region should it be descended?"

उत्तरेणावकाशोऽस्ति कश्चित्पुण्यतरो मम || २-२२-३१
द्रुमकुल्य इति ख्यातो लोके ख्यातो यथा भवान् |

"Towards my northern side, there is a holy place. It is well known as Drumatulya, in the same way as you are well known to this world."

उग्रदर्शनकर्माणो बहवस्तत्र दस्यवः || २-२२-३२
आभीरप्रमुखाः पापाः पिबन्ति सलिलम् मम |

"Numerous robbers of fearful aspect and deeds, having the sinful Abhiras as their chief, drink my waters there."

तैर्न तत्स्पर्शनम् पापम् सहेयम् पापकर्मभिः || २-२२-३३
अमोघः क्रियताम् राम तत्र तेषु शरोत्तमः |

"I am not able to bear that touch of those wicked people, the evil doers, O, Rama! Let this excellent arrow with out vain be released over them there."

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So, Shri Rama did not want the invoked DIVINE WEAPON go waste, though he knows how to withdraw it.