They can stack from procs (which is the intent), and I've used a couple of builds which verify that it can work. But you're right that there are definitely some cases which are broken. I'm aware of at least one with arc blades which causes the channeled attack to only stack dots from certain sources and not others. My guess is there are plenty of others. I can't solve this for us, obviously, but it might help to describe a couple of specific, concrete scenarios that you've run into so that it's a bit easier for the devs to track down the bugs.