My impression was that on fact the Assyrians suffered from several main issues interconnected :
- overextention
- the deep hate of the conquered populations due to their brutal treatment and deportation
- the "colonial" type of exploitation of the conquered lands for the benefit of Metropole
- the dispersal of the loyal native Assyrian population trough the Empire
- the increasing population in Assyrian land which was alien, diverse and held no loyalty to Assyria/do not identify themselves as Assyrians. This was cause of deportations.
- the lack of efficiently assimilate the elites of the conquered countries and of their populations
- not an emphasis on converting people to Assyrian religion
- the decrease of the military base of recruitment.
- passing from militia type to a professional standing army tremendously improved the Assyrian successes but if one or two armies are lost then it became terribly exposed.
- reliance on very strong leaders
- way too many enemies and 0 allies
- bad luck at the end