Congrats! That's how they're supposed to work!

I'm concerned with your "not perfect, but getting better" comment. It really should be "perfect"... If it were mine, the next step would be to disassemble the magazines, clean the internals of the mags and reassemble paying close attention to the "orientation" of the magazine spring and follower. Look online for a diagram on how they go together.

I use "Hornady gun cleaner and dry lube" spray for this and it works well.  If the problem still exists, then replacing the magazine spring would be next. A weak mag spring will lack the tensile strength to lift that last round.

It sounds like a combination of issues. If the slide stop was installed wrong in conjunction with weak mag springs, well, that would be a difficult diagnosis. But if you correct them sequentially, you should be able to eliminate the variables.

The slide stop spring is compressed against the top pin. The bottom pin passes through the slide stop lever. If one places the slide stop in place, then inserts the bottom pin before installing the top pin, then when you do insert the top pin, the "spring" will not be compressed against the top pin.

It seems like a trivial thing, but it causes a lot of new Glock owners much grief, exactly as you describe.

I'm new here and late to "this party"... Sorry to hear about your feeding problem with your Glock 20SF. I would offer (2) suggestions. First, remove the pins associated with your slide lock and the slide lock will be easily removed. Inspect the "spring" on the slide lock, if it is intact, then it "should" still work. Then re-install in this order.... the top pin, then the slide lock lever with the spring below the top pin, and then the larger bottom pin.

You may need to manipulate the slide stop a little as you push the pin in, once it's lined up correctly, the pin should seat pretty easily. It's a common mistake to install the slide stop lever bottom pin first which leads directly to the problem you describe.

If that is not the issue (the slide stop is installed correctly and problem still exists), try disassembling and give your mags a good clean with some Hornady gun cleaner and dry lube.

Your issue sounds to me anyway, to be either the slide stop lever or a magazine issue. I'm new here, but I'm pretty familiar with Glocks.

I just wanted to say G'Day! I recently purchased my first 10mm, a Glock G20SF. Like most folks here, my only disappointment is that I didn't get into 10mm sooner!

I'm a reloader, so ammo cost was never a factor, but oh we'll better late than never. I've "lurked" a bit here before signing up, and recognize a few folks from other sites. Even so, I think I'll stay as long as you'll have me! LOL!

I look forward to learning and sharing about our common interest in an interesting small slice of the firearms world.

Cheers!

Ed