Problem with Campagnolo 12s Super Record Left Shifter
 

I am installing a brand new 12s Super Record Group set. The left shifter is shifting the chain up to the large chain-wheel, but it doesn't 'click' into place. i.e. It doesn't hold the chain on the large chain-wheel. As soon as I let go of the lever it shifts back to the small chain-wheel. I'm sure this is a simple fix

Off the cuff, check both cable length and outer limit screw.

It's possible that the cable is causing the shift, but not allowing the levers cam to engage the latch. If so, slightly slackening the cable will resolve the problem.

I had this happen on a newly installed 11sp Record. It's embarrassing to admit that I had the cage cover installed upside down and so it was interfering with the cassette but only slightly...enough to cause the same thing you are experiencing! lol

Every time I've run into this I've found the shift cable is too tight. Loosen the cable slightly and the lever clicks in.