Why did the Byzantine emperors easily recognize the imperial title for Charlemagne and his descendants, but in the XI century

Why did the Byzantine emperors easily recognize the imperial title for Charlemagne and his descendants, but in the XI century they did not recognize this title for the heirs of Otgon I?

In the era of Charlemagne in Byzantium there was a turmoil – iconoclasm. Under these conditions, the Eastern emperors had no time to interfere in the affairs of Western Europe. At the end of the 10th century, Byzantium flourished and waged aggressive wars. Therefore, the emperors of the East recognized the title of Otgon I (the winner of the pagan Magyars) for his son Otgon II and grandson Otgon III, but did not recognize their successors from the new dynasty – the Swabian.