Good advice above. Try loosening the bolt completely. Is there play now or is the bar still in with a nice snug fit? If there is play, there's a good chance that a 25.4 bar snuck into a 26.0 stem. Beer can shims work wonderfully to solve that. (Most of the time. All three, bars stems and beer cans are made to tolerances and vary,)

I've used those shims for some runs of years. I cut rectangles exactly the width of the stem with good scissors. Very hard to see.