Campagnolo Chorus 12 indexing issue

Having issues trying to index brand new chorus 12 speed on new frame, when on large from ring going from 12th down at rear first 4 shifts work fine then after that chain wants to jump up to next ring when I adjust so it runs ok but it then fails to shift from 12th up again as loosened to much anyone have any ideas?

Bent derailleur hanger, worn chain, worn cassette?

Also a dirty or kinked cable?

new frame new group set never been out of the house

Check hanger alignment

Is the cable clamped down correctly? It can make a difference.

gave up and sent it to local bike shop. I worked as a bike mechanic in 1990 at on-yer-bike in London Bridge but this got the better of me after 3 hours. First 12 speed set up I've worked on.

Check hanger alignment

This would be my guess too. Always check the alignment, even on brand new frames.

First 12 speed set up I've worked on.

They’re fickle things. Let us know what the issue was?

Hope to get the bike back on Tuesday will let you know what the problem was.

Turned out I'd put the cable connecting to the derailleur the wrong side of the connecting bolt I'd put it along the top not the bottom. I'd taken the bike to Rhythm and Bike in Brighton they tried not to charge me but I gave them a drink, great guys.

At least it was a cheap fix. Thx for not leaving us hanging.