I just don't know why I can't get this achievement, even though I'm pretty much sure that I made a pile of even more than just 20 items (twice, in two different places of the same location). I want this achievement, but just can't get it.

What am I supposed to do? How to fix it?

It's hard to debug what went wrong without seeing the pile and what actually recycled and what did not. I take from your 'pile' that it's really a RC you are using and not a "stationary" Recycler, which doesn't count. When I pile up stuff, mostly destroyed operators, which give about 0.77 mineral and 0.13 synthetic each, and the pile is to large/wide/uneven, some of the items are not recycled, even though they are inside the blast radius. Some are even pushed away afterwards, so it's not obvious that these items where part of the pile.

This achievement is therefore best done in the "Hardware Labs" Recycle Sphere in "Ballistics Lab". It gives you a very controlled environment and nothing can get away from the blast.

After siome trial and error, I finally got it. You have to get at least 20 different types of junk objects. Not food, wepons, etc. If you are short of inventory spaces, you can put them inside a recycler (and not recycling them of course) and wander arond te level searching for more junk. When you get the 20 junk objects (they have occupy different 20 slots, if an item has 2 or more, it doesnt count), just pile them up and throw a recycler charge, after it's done, the achievement should pop up.
Hope it helps :D

So, a little update for you, folks.

Just launched the game as usual today to try get this achievement once again, and suddenly... It worked.
I finally have this achievement now.

Can't really say if it was because I reinstalled the game, tho not because I uninstalled it in the first place, but that's the another story.

Anyone struggling to get this achievement to this day - try to reinstall the game, maybe that would work.
I guess that worked for me, at least.