This is a traffic-shaping/port-blocking issue, and can be caused by the people you play with, rather than just by your own connection. I discovered this when a friend of ours started playing Destiny alongside myself and the rest of my usual clan. Latency-wise, he's on an identical connection to my son, (who physically lives less than an hour from him.) They're on different ISPs, however, and the one in question is famous for traffic-shaping pretty much any peer-to-peer app during peak hours. Literally, as soon as this guy joins our fireteam, all of my melee skills stop triggering. As soon as he leaves, they start working again. Bungie probably never knew it was a problem, because they've the common sense not to use awful ISPs. Edit: for those mentioning lag-switching, yeah - that can cause it too, because in essence, the switch is just another throttling technique, like traffic-shaping.