I used to roll cast miserably, no matter the rod or line size. I joined a fly fishing club which conducts a twice-yearly "Casting Clinic." In less than 10 minutes, my roll cast went from 20 feet to 45 feet, just by having an experienced fly caster give me a few tips. Having somebody help you is very important, or at least was for me. It's been a decade or more since that casting clinic, but I remember him saying that you need a belly of line, some of it slightly behind you, as you raise up the rod. As you push the rod down, you accelerate as you go, so up until the end, you're increasing your speed. This helps to load the rod - keep in mind, the rod in this case is, essentially, a spring, and if you don't compress that "spring" by loading it, it won't propel the line out. Oh yeah, when you finish the cast, it should be an abrupt halt, to let the rod straighten out, pulling the line along, and your rod should be +/- horizontal, pointed at where you want to go.
That's what I learned, and it works great.