I was just training on a practice dummy today, and after I activated my first DT I haven't been able to build up any SI charges since then. It just hangs at 3 charges if that much and just stops. Has something changed?
This sounds to me like a bug. I can stack DT fine, so I don't know what the problem could be. Perhaps a client restart, assuming you haven't already done so, and if that doesn't work, maybe contact a GM or use repair tool?