The epic states that initially, the battle was even between the two foes, situation changed when Karna's chariot wheel became stuck in the mud due to a curse he received from a Brahmin.
The epic states that initially, the battle was even between the two foes, until Arjuna floods Karna with arrows after Bhima reminded Arjuna about his oath. Then Arjuna resolves upon killing Karna and Karna's chariot wheel became stuck in the mud due to a curse he received from a Brahmin.