OVEREXPANSION Rome had all of the land from the Atlantic Ocean all the way to the Euphrates. They had a lot of land, which made it hard for the Empire to communicate and defend themselves from local rebellions. Eventually they ran out of money (kind of) making all of their new advancements slow, tearing Rome apart forever.
Christianity When Christianity began to spread, it gave people a sense of hope, making them want to follow this new religion and forget about their Roman customs. Eventually Rome would not have enough followers, making it hard to please their "god's" and keep the empire successful.
Rise of an Eastern Empire When the empire split into 2, East and West, they became distant, creating a conflict between the two over who got what resources, how the military would be controlled as well as religion and government etc.