Would you be able to take a screenshot or video of this happening? Just from the initial sounds of it, I'm at a little bit of a loss as to what might be going on.
Sometimes their armor is really good at absorbing blows. It could be a case of that. Does the behavior change if you jetpack-attack them? I'm wondering if the attacks just aren't landing correctly, or they aren't doing the damage that they look like they should be doing. - I'm guessing this issue is intermittent? Again, a video would be really helpful. Programs like FRAPS, Plays.TV, Radeon ReLive, and Nvidia Shadow Play are some good options on that front.
-Cheers