Justinian II sent the canons west for the pope's signature and Sergius I refused to give it. The emperor sent an officer to arrest him. The imperial troops stationed in Italy mutinied, took the pope's side, and the officer is said to have ended up hiding under the pope's bed. The canons were never received at Rome.
The refusal was not procedural. Several canons contradict Latin practice directly and were understood at the time to be aimed at it. A married man may be ordained and continue to live with his wife, which the western church had been moving away from for centuries. The Saturday fast that Rome kept in Lent is forbidden. Christ is to be depicted in human form and not as a lamb, which struck at a western iconographic convention. Read from Constantinople these are corrections; read from Rome they are one church legislating for another.
The two families of churches have never fully resolved it. The Orthodox churches count these canons as belonging to the sixth ecumenical council and binding on the whole church. The Catholic Church accepts some of them as ancient discipline, does not treat the council as ecumenical, and holds that canons never received in the west never bound the west.
This is the clearest first millennium case of a council which is law in one half of Christendom and a dead letter in the other, and it is a useful corrective to the idea that a council's authority is settled by the fact of its meeting. Reception is what settles it, and reception can simply fail.