No, that's probably not enough. 3/4 of a cup of sugar would have been about right, but you need more if you used DME. Typically, about 1 and 1/4 cups is about right.
All is not lost -- you can re-prime. Here are some instructions (copied and pasted from a recent thread):
You can add new sugar to the bottle if you have access to carbonation tabs. The Muntons tabs are better because they are small and you can control your carbonation level by selecting the correct number of tabs. I don't like the Coopers drops because they are too big to allow good control over how much sugar goes into the bottle.
Before you add your carb drops, you MUST get rid of any existing CO2 in the bottles. Otherwise, when you place a carb drop in the bottle, it will cause a major gusher (you can guess how I discovered this). Make sure the bottles are at room temperature, then shake them gently to drive out some of the dissolved CO2. Pry open the cap just enough to vent the gas, and repeat (you may have to do this more than once). Before you take the cap right off, chill the beer down in the fridge overnight.
Once the beer is cold, it won't foam when you put the drops in. Working quickly, take the top off the beer, add your drops, and replace the top. A spray of StarSan on the mouth of the bottle would be a good idea, too.
Once the beer is capped, move to a warm spot for 2-3 weeks and enjoy!