The variances in slide stop spring pressure certainly will affect whether the slide auto-forwards. For me, however, the key determining factor has been the amount of friction between the slide stop and the slide notch.

Lots of friction = a stop that is difficult to disengage, even with a relatively new or stiff spring. Often new stops, commonly a stamped metal part, will have a fairly sharp ridge that engages the slide, making them most impossible to release unless the sling shot method is used. When the mating metal surfaces wear, reducing friction, auto-forwarding is more likely.

Most slide stops on semi-autos work the same way, the new 2.0 M&P being a notable exception.

So, here's the fix:

You don't want it to auto-forward? Use some 400 grit wet/dry sandpaper and lightly rough up the two mating surfaces.

You do want it to auto-forward in a manner you control? Polish the stop's engagement surface first to reduce friction, just a little at a time. Add some roughness if you screw up, or buy a new (cheap) stop. Avoid working on the slide's notch, although the same polishing principles apply.

For me, a properly adjusted slide stop will function in numerous ways to release a locked slide (Thumb, finger, slingshot, auto-forward). This means I polish the stop engagement surfaces on all my pistols of all makes before I ever shoot them, and I have been doing so for over 25 years. They all function the same, and I control it, not some haphazard factory slap-together procedure with varying spring pressures, tolerances, and amounts of friction based on mass produced parts and speed of assembly. All custom guns have adjusted slide stops.

Training is the issue more than mechanics. Any procedure or mechanical part can fail. You must have an ingrained plan "B" ready to go as a backup. Whatever method you choose as your primary for releasing a locked back slide, you need to train for an alternate. If you release the slide and expect a bang and get a click, tap-rack-bang immediately. The problem could be any of a number of things, with not picking up the top round from the mag being just one of them.

If TRB, doesn't work, go to your next trained procedure, usually a complete reload.

I have found auto-forwarding to be very fast, simple and reliable for me when my stop/slide interface is adjusted correctly. It is my primary means of releasing my slides from lock. However, I use other methods to release a slide to stay familiar with them and as a backup.

As far as the claimed reproduceable failure to pick up the top round in a downloaded mag, I have never had it happen. Running the mag home hard seats it positively--a good thing. This seating of the mag jars the slide enough that the stop immediately drops. The laws of physics seem to favor the idea that the rounds in the mag would want to continue in the direction they were traveling, i.e., upward. I don't see how they would do the opposite, i.e., compress down into the mag so the slide would not pick up the top round.

On the other hand, if in a two-step procedure, the downloaded mag is inserted and locked in place with the slide open, and THEN the gun is brought DOWNWARD and hits a hand or other object at the base of the grip to jar the slide stop loose, that could send the rounds down into the magazine and the slide could miss the top round. This wouldn't happen with a fully loaded mag, or a mag spring stiff enough to prevent such compression, either from being new or because of having enough rounds in the mag to prevent it. It also doesn't happen with the single step sharp insertion method I use.

Just because I have not experienced the problem does not mean others have not either. I look forward to being educated with some logical explanation.