my LC9S pro was the same way (before i sold it of course). the issue is the way ruger cuts the notch on the slide. the angle of the notch cut is such that it puts upward pressure on the slide stop. it's near impossible to overcome this with a thumb by pushing down.
probably the easiest solution is to just leave it alone and learn to slingshot the slide. alternatively one can take a few gentle swipes with a diamond file to round out the edge a bit and give it a more "forward angle". this will allow downward pressure to drop the slide. the issues with this are 1) slight removal of the cerakote/nitron finish and 2) if done too much, the slide may drop too easily like with a nudge, bump or inserting new mag.
if i had kept the LC9s i would have made the modification, but once I felt a G43 I had zero use for an LC9s anymore so it got sold.