[4V50 Gary]

The slide stop is activated by the magazine follower. When the magazine is empty, the follower pops up and should engage the finger of the slide stop, pushing it up to intercept the slide in its forward return.

Check to ensure that the slide stop moves up and down freely.

Check with each empty magazine that the follower engages the slide stop in time to lock the slide to the rear.

It could be that on firing the last round, there's not enough energy (recoil buffer slowing the travel too much) in the slide such that it retracts fully. If this does not happen, then the time allowed for the slide stop to pop up is shortened and the slide goes forward and closes. In other words, the slide is moving forward faster than the slide stop may be pushed up by the magazine follower.

Your magazines springs could be weak or the recoil spring on your gun too strong. Too much energy is expending in unlocking it and when it does, it doesn't have enough energy left to push the slide back far enough (perhaps enough to extract and eject, but not for the slide stop to pop up). It could also be the recoil buffer which is slowing down the slide too much.

Then again, if we're talking energy, better look at your ammo. It could be that the current Winchester 185 grain isn't enough.