I had same issue with mine and figured it out. I upgraded mine with the apex tactical trigger and spring kit with a 20lbs slide spring and stainless steel guide rod. I posted a video on how to do it on youtube. The issue is the top of the spring on the release lever moved back slightly and the end poked out just underneath the block enough to restrict full movement of the release. As a result the slide merely ramped over the release. At first I thought the arm was contacting the block but when I put my magnifiers on there it was plain as day. I may have not had it placed fully correct when I put it back in after the mod (most difficult part of mod) so thought maybe the same thing happens at the factory. I doubt they are assembled by machines and that bugger is tricky wicket getting it in and aligned so you can put the pin in even with the tool they give with kit. Used a jewelers screwdriver to get behind spring at bend and pry it gently to the front so it sits just in front of block ledge and not under it. Stays every time now although I just did it today and haven't field tested to be 100% sure that was the cause but you can see lever sits in position as it should now. Good luck
Smith&Wesson sd9ve slide won't stay open simple fix. - YouTube