I recently started reading a book by Cam Rea called Leviathan vs. Behemoth: The Roman-Parthian Wars, 66 BC-217 AD and I realized that despite the successes the Parthians had at Carrhae in 53 BC, and Antony's Parthian War (40-33/20 BC), the Parthians tended to end up on the short stick during the various Wars in the AD period.
Rather than a typical What if they were more successful, I'll go on the bat and say, why that was the case. Why did Parthia ultimately fall apart in the wars to come later on, and could anything have been done to prevent this?