Hmm, it might be a bug in the current version. I will try to see what caused it and fix it. Do you, perhaps, have a clip or something so I can see it better?
I did not have a clip, however I just tried to recreate it and it seemed to be working fine. I almost wonder if it had something to do with the upgrades I had chosen, I had Orbital, Freeze, and armor shred all levelled somewhat high along with bullet speed of 12x. It happened that the worms were making it through Orbital (which is somewhat normal) and then they would make it right next to me, no longer moving, and the shots would be raining down on them but doing nothing as they chipped away at me. Eventually the game would allow damage on them after I would get hit 3 or 4 times from each.
I noticed the first time that it would eventually kick in but this time around it didn't even register once which was odd, this was on HTML (Google Chrome specifically) I may try it on Android at some point as well, I can give feedback if I notice it there as well, but maybe it's just a browser based issue.