I've had this happen to me alot so a few things I've noticed:
- Terran difference: I.e if they are on a ledge it won't go through, or you'll go flying over their heads if coming off of a ledge yourself
- Holding a direction: Sometime I've notice that when I hold forward to grapple and they dodge to the side I'll fly right past them. This counts for strikes too. I've hit an elite only to go flying toward the mob behind him and got grabbed.
Getting knocked out of the grapple can cause them to fail too. The 5th Council and their shot gunners highlight this perfectly.
Hopefully this helps explain a bit about what is going on.