One of our members, a fantastic guitarist, Rick Fielding, is a tuning fanatic and prefers Golden Gate on 12 but also uses Shubb and Kyser. I use both Shubb and Kyser myself. It is inevitable that any spring load capo will weaken after time, but if you "fit" the capo to the guitar and slot the rubber on the capo on the E and A "big" strings, you'll have a lot more success. In another place, Rick made this comment and explains what I am saying:

Best way to capo a 12 string is to cut two tiny "V" shaped grooves where the "Big" E and A strings contact the rubber. This works! I'm a tuning fanatic, and never had any trouble after I started doing this. Personally I use a Golden Gate capo for 12 string, but it works well with Shubb, Dunlop "C" clamp or Kaiser.

Also the arm of the capo should match the radius of the neck which is constant all the way up. Doing a little work on a Shubb will make it work infinitely better and last longer too. The inconvenience of most clamp type capos makes this a better option.

Spaw