If it's anything like this one Lickbike.com | Dimension 26.0mm adjustable road stem - 110mm) :
Unscrew the bolt a little, enough to make it looser but not enough to adjust the angle. Put some weight on the bars just like you were riding, push in the cap on the far end of the bolt head to make sure it's not loose, then tighten the bolt as much as you can with your hand.
At least for mine that took out the ability to pull on the bars to make it click.