I had an issue with the same brifter, 8 speed Campy. The LEFT one would not " return" to the small chainring.
I did not want to do the work myself being that I do/didn't know how to fix and what was correct.
I ended up sending it to Branford Bike in Seattle.
Pat was the one I spoke to before I sent it out.
He is the person who repaired it. VERY NICE guy. And VERY knowledgeable !!
I had it back in about 8 days.
Works Prefect !!!!!
VERY reasonable , and Pat is an expert with Campagnolo !!! Especially 8 speed Brifters !!!!