Achievements didn't pop

[Brian]

I did 100% of the game and now checking what achievements I'm missing I realised that I don't have "Beam" and "Momentum", even though I did that part of the game and I believe I saw them pop at the time.
I was never offline, but maybe Steam servers were down when I got them? Is there a way for me to retroactively get them? Or what exactly triggers them in game, so I can go try again? I just played the game to 100% and don't want to start over on a new file... The other 5 achievements I'm missing I believe I can stil get after I completed everything.

[Mehhh]

I was in the same boat last year. In my case, I think it was the achievement for getting the purple gun. And as far as I know, it was because the game couldn't contact the Steam servers at the time.

On the downside, there's no way we know of to retroactively get it. On the upside, playing the game again proved to be fun enough that I even did an entire new 100% playthrough. (It helped that David had patched the game with a few new features I wanted to test.) You save on all the time you spent figuring out the puzzles the first time; it goes by pretty fast.

So the short answer is "go ahead and do a new playthrough".

[DavidM]

If you saw them "pop" they did register. They don't pop up if they don't.

Momentum is when you get over the abyss with the ball in your hands and put it in the ring on the other side.

Beam is getting the beam, which is of course mandatory.
Not sure what happened there.

[Dedlok]

I got Momentum after I used a Supraball jump to cross the cliff by the Diamond Ranked Supraball guy's house. Don't recall whether or not if I put a ball in a ring afterwards, but it definitely got it shortly after making it over at least.