So I'm just trying to round up the last few trophies before going on to Hardcore mode and the last one I'm missing is "C-Section". It says I have to knock down an enemy with the Contact Beam Alt-fire and then kill it before it gets back up. 
 
However the alt-fire doesn't seem to do anything at all. Enemies are completely unphased by it. Isaac does his little ground-pound thing but nothing happens. It doesn't seem to be doing any kind of damage either. 
 
I haven't actually used the Contact Beam in any of my 4 first playthroughs so I don't really know how it's supposed to work.

How close are they to you? When in range the contact beam secondary fire should knock them down and even damage them.

Even if they're up close it does nothing.

Weird. I guess you should try quitting out and going at it again. If that doesn't work, your game is just broken in an oddly specific way.
 
Make sure they get right up on you, just in case.

I don't know if it should matter but what enemies are you using it on?  Maybe try upgrading the alt nodes.

I've tried on a variety of different enemies. Most of which have been on two legs.  
 
I've tried with a clean one, both the regular and the hacker bonus version. I've tried fully upgraded on both as well. 
 
If I put a node in special, the stasis proc works. 
 
So far looking online I can find no other cases of this happening. I'll try and contact support and see if they can shed some light on this mystery. I don't understand how a bug can be so specific if I'm the only one experiencing it. I'd take video but can't capture on my PS3.

I've even tried crying in a corner. That didn't work either.

That's pretty crazy. I guess you get to be special at least? I wonder how a glitch like that can be so contained.
 
Strange stuff dude. Hope it gets sorted out.

Just tried making a new ng+ and i finally got it working. However it seems to only work less than half of the time. It's a bummer, but I don't like that weapon anyway. 
 
TROPHY GET!

I got mine eventually, but it did take quite a few tries. I got mine by selling my fully upgraded CB and buying it again, worked first time then.