Rome had always had its fair share of bad emperors. Caligula, Nero, Commodus are such examples. But there always followed good emperors to correct their errors. At Rome's end however, there was really no good rulers. For example great generals like Stilicho were killed because the emperor feared losing his throne to them. If you kill your best generals, your army will never be as good as it could be.
Also important to the weakening of the Romans was that they began to get more and more civilized. Civilized people were not as good warriors on the battlefields as barbarians. For a long time the Romans had their own barbarians. Gauls, Illyrians, Belgians, Helvetians, Germans, etc.; they all joined the Roman legions. But soon they too were living like Romans. They enjoyed the fruits of Roman civilization and hence became less barbarous, - and less able to fight the barbarian Goths, Vandals, Sueves, etc..
Another important reason is perhaps that of Roman unity. The early Romans who built the empire stood united. By the end of course Rome had two capitals, Rome and Constantinople, each with its own emperor. But to the earlier Romans, Rome had been something special, something they served. But the later Romans only sought power for themselves. Almost every military commanders secretely had his eye on the throne and was ready to overthrow the emperor and take power himself. The armies spent a lot of time fighting each other for power in Rome, rather than fighting the enemy.
To this cataolgue of troubles one needs also to add economic problems. Rome was spending more than it could afford. The free food rations for the poor of Rome and Constantinople were costing a fortune. The Purchasing of exotic spices and silk from the orient ment that over time Rome was spending its gold on overseas luxuries. Gold which didn't return. Soon Rome didn't have enough gold to produce coins with.
And then there were the plagues. Deadly diseases brought in from the east swept across Europe, killing vast numbers among the population of the empire.
Even if the mass migration of the wild, Germanic people of the eastern plains is no longer seen as the all-important reason for Rome's demise, it naturally still is seen as one of the capital problems which caused the empire to fall.
As a last reason I'd also see the disastrous decision of emperor Valens (who was emperor of Constantinople) to allow the Visigoths to settle in Moesia.
The Roman empire's borders to the north had been the river Rhine and the river Danube. They are wide streams and therefore hard to cross. This means, the border was easy to defend. But when the Huns from the east attacked the Visigoths on the other side of the Danube, emperor Valens let them settle on the Roman side of the river. True, the Visigoths then were friends of the Romans. But that soon changed. It didn't take long and bad living conditions, poverty and starvation led the Visigoths to revolt. In the following war, the Romans no longer had a protective river from behind which to defend themelves. The enemy was settled within their own empire. - And it should be the Visigoths who eventually sacked the city of Rome itself.
So, the main points for the fall were
1. bad emperors
2. increasing civilization of the people of the empire (which means weaker soldiers)
3. Roman disunity, endless infighting
4. economic decline
5. plagues
6. mass migration
7. and the settlement of the Visigoths in Moesia
