300 watts demands more from your
batteries than they (and the mod's board) can provide. This isn't too surprising since there aren't any 18650s that can safely provide more than 270 watts in a three battery mod, and there aren't many that can be safely used at that level. Unfortunately, when a manufacturer says their mod goes to X watts they mean that the board can provide X watts (at least at certain resistances); it doesn't mean that there are any
batteries that will work well (or at all) at that wattage.
What battery make and model are you using? Depending on what you have now, I might be able to point you to some that would work better at around 250 watts, but you're not going to get decent performance above that with current battery technology.
Oh, and the source of the problem is probably your mod's voltage limit (10 volts) combined with the lack of boost, the resistance of your coil and the limitations of your batteries.