My stem won't tighten

I've just fitted new bars which needed a 26mm stem [my old one was 25.4mm];

I purchased a 2nd hand one off the 'Bay, fitted everything, but stem won't tighten fully so it still moves about slightly. Arghhhh!

What am I doing wrong if anything?

Change it (the thing) to not the old before or swap it (that) around for different.