"New old stock". Do you mean it's a used pistol?
If it's a NIB/never-sold example I'd check the magazines for congealed shipping oil inside that might be present from the factory and give them, and the pistol, a good cleaning and then check for function again.
Generally, if the slide stop lever isn't lifted by the follower when the slide is manipulated with an empty magazine in place, the first things I'd check would be for the magazine to be properly inserted with the mag catch locking it in place; the magazine for any debris/fouling/old oil sludge which might be interfering with the follower's freedom of movement; the follower for any visual damage/excessive wear where the lever's extension engages the follower's shelf; and the tension of the magazine spring.
It's not uncommon when a magazine spring starts to become too weak for normal functioning to see occasional (with increasing frequency) failures-to-lock when the slide is cycling under fire or even when manually cycled, and then feeding failures occurring (usually starting with last round issues, or the last couple or so rounds).
Usually a new magazine spring/follower will resolve some of these issues (the current followers are light blue, BTW, and the last dbl column full-size .40 mag springs I received for 4006TSW's were also painted light blue on approx the bottom half).
It's also possible for the slide stop lever assembly (it does contain a spring, plunger & roll pin, you know) or the side plate to be damaged, too. The plunger & spring on the .40 slide stop levers has been revised over the years. I discovered that when an older 4006 was brought to me for repair from another agency one time.
Just some thoughts where I'd look ...
