The Slide Stop Lever has a spring. With the Magazine and Slide removed, does the Slide Stop Lever snap down if you push it up? If not, the spring may be broken, resulting in intermittent behavior of the Slide Stop Lever.
Also, note the correct position of the Slide Stop Lever Spring, fitting under the Locking Block Pin. If you have dismantled the Pistol, it's possible that it was re-assembled incorrectly. Let me know if that spring is out of place, and I'll give you the steps for re-assembly. Order is critical.
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