What is the reason for the instability of Charlemagne’s empire?

During the reign of Charlemagne, many territories were conquered, the borders of the state were significantly expanded. However, the empire was unstable, because the state turned out to be fragmented, there was no state administration apparatus that would be effective. The peoples spoke different languages, there were no clear economic ties between the regions. This did not allow the creation of a single state. A lot of effort was spent on guarding the country’s borders, preventing their capture or secession.