i was running 25/30/-/15/- and it was bad. maybe im a sucky player, or a "baddie" as some [DnT] folks would consider but i mean it was not enjoyable. i was dying from aoes, environmental dmg, dots, and i was avoiding 99% of all direct hits from bosses. there are a lot of uncontrollable factors in higher level fractals that if you dont have the utility or the sheer defensive stats to compensate, you end up more useless to your team than the 20% extra dmg output from going full dmg spec.
It probably has nothing to do with you. When you get to 30+ fractals your dps players are only as effective as your control players enable them to be. If your reflects arent good, if your focus pulls suck, if your guardians dont use stability and ring of warding effectively (to name a few things) your dps players are going to be limited. Without effective control, bursting down trash is much more difficult and encounters will be prolonged and the inevitable splash damage will occur.
So yeah, it probably isn't you as a Thief.