I think the question is ill-posed. The Romans were "win some, lose some" throughout their history. Look through this list of Roman battles. Even at the height of Roman power, they had huge defeats at battles such as the Teutoberg Forest. And even in the last crumbling of their power, when Rome had already been sacked once by the Goths, they were still able to repulse Attila the Hun at Utus and at Chalon.

What changed over the centuries, rather, was that the Romans became less willing to call up new armies again and again after their defeats. They slowly devolved this responsibility onto non-Roman auxiliary forces, who ultimately demanded independent political status that led to the breakup of the Empire.