3. Un-Loyal Army
Since they didn't have enough money to fund their own army, Rome hired Germans to fight under Roman commanders. However, they weren't very loyal to the empire. This is third because although it is a problem, it's not as big as the other two.
1. PLAQUE
Not only did the plaque kill a lot of people, it made many people sick, and sick and dead people can't do their job. That means less goods are produced, there is a smaller army etc. This is number one because it was a huge problem that led to many other problems.