So that's it. It took me 14 hours approximately to collect everything in the game - from all pollens to outfits and tinykins included - but achievements did not trigger at all..

That was frustrating, ofcourse.
If you do some research over the internet you'll find out that apparently achievement progression is reset once you quit the game.
This means you have to enter every room again to unlock such progression achievements.

Another thing I realized is that I was missing two achievements, such as "Gotta Catch 'em All!" and "Sniper".
I completed them, but achievements did not trigger too.
Solution was to enter the last room - the attic - and achievements unlocked as soon as entered the level.

I hope these little glitches will be fixed in an upcoming update, but in the meantime I hope this will help you as well.

Cheers,
Antoine