There's not enough info in your post to troubleshoot the problem, or to suggest replacement part options.

Specifically what kind of Campagnolo shifter and derailleur? Model? Age? Number of speeds? Front or rear?

Is the problem at the shifter or derailleur? If you're not sure, grab a section of exposed cable along the downtube of the bike and see if you can pull cable by hand to move the derailleur. Does it move when you pull? Does it quickly return to normal when you release?

And if you could post pictures, it would be really helpful.