Why could Karna not save Jayadratha on the 14th day of war from Arjuna, if he was a match for him?

Why could Karna not save Jayadratha on the 14th day of war from Arjuna, if he was a match for him? Well I think that you know about the van parva account where Jayadratha tried to abduct draupadi and was defeated by the Pandavas. Due to this Jayadratha desires to avenge his humiliation from the Pandavas by defeating them in the battle field. So, Jayadratha performs a rigorous tapasya in order to please Lord Shiva. When Lord Shiva appears before him, Jayadratha in turn asks for the ability to defeat the Pandavas. Shiva replies that such a boon is impossible, as the Pandavas are too mighty. However, he gives Jayadratha the ability to check the advance of the Pandavas and their forces for one whole day. However, Shiva warns Jayadratha that Arjuna is an exception to this boon. The messages of the encounter are manifold. One, that Lord Shiva is known to love all of his devotees equally and does not ignore the tapasya of anyone. Two, that God must return prayer with something. Three, that God, or any giver of boon, should only promise what is within his limits. Naturally, Jayadratha fights on the side of his brother-in-law Duryodhana in the Kurukshetra War. On the 1st day of the Mahabharata war, in the noon he defeated King Drupada. On the 13th day of the Mahabharata war, when the chakravyuha is launched by Dronacharya, Jayadratha makes use of Lord Shiva's boon. Arjuna's son Abhimanyu manages to enter the formation; he intends for the Pandava forces to follow after him and smash the formation from the inside. Jayadratha moves to close the gap, and is able to hold all the Pandava brothers and their forces at bay. As part of Drona's strategy, Arjuna and Krishna are busy battling Susharma and the Trigata Army elsewhere. Abhimanyu, who does not know how to exit from the chakravyuha, is trapped and brutally killed by the Kaurava Warriors in a combined attack. The Pandavas are startled after finding that Jayadratha was able to hold the world's most powerful warriors at bay. In particular, Draupadi, Yudhishthira, and Bhima feel very guilty for not killing Jayadratha when they did have the chance. Arjuna blames Jayadratha to be the cause for Abhimanyu's death. He vows to kill him the very next day before sunset, failing which Arjuna would kill himself by jumping in a pyre of fire. This sets the stage for the epic 14th day of battle. Arjuna's revenge Dronacharya arranges a combination of 3 vyuhas in order to protect Jayadratha from Arjuna. Jayadratha's head falls in his father's lap Bhima, Satyaki and Arjuna tear through the Kaurava army. At a climactic moment, with the sun nearly set and thousands of warriors still between Arjuna and Jayadratha, Krishna sends his Sudarshana Chakra in order to mask the sun and create an illusion of sunset. The Kaurava warriors rejoice over Arjuna's defeat and look forward for his imminent suicide. Jayadratha was hiding behind Duryodhana, relieved that he was saved. Therefore, he came out of the formation. Suddenly the sun was free from the eclipse and Krishna tells Arjuna, pointing at the sun that the sun had not set, but it was only a solar eclipse. He then pointed at the hiding Jayadratha, telling Arjuna to sever his head. He then tells Arjuna to shoot the head into the lap of Jayadratha's father. Arjuna then quickly picked up his Gandiva and shot a Pashupatastra at Jayadratha. Therefore due to this karna was unable to save him from arjun. Jayadratha's head than flew away very far and landed on lap of his father, Vridhakshatra. His father, being a sage had granted him a boon that whosoever will be responsible for his son's severed head to fall onto the ground will have his head burst into 100 pieces. Therefore, when his father, horrified at having his son's head in his lap, hurriedly got up, the severed head fell to the ground, killing Vridhakshatra at the same moment.