low carbonation causes the beer to seem watered down. if you get it to fully carbonate then this will probably fix that issue. i think you did these things:
1. you didn't let it carbonate long enough. let it go for 4 weeks.
2. you didn't mix the sugar in well enough. some bottles will be flat, some will be so overcarbonated that they are ready to explode.
3. you bottle conditioned at too low a temperature, 65 degrees or lower
to fix it, try this: turn all the bottles upside down and shake them for a few seconds to "rouse the yeast". let them condition for another two weeks in 70 to 75 degree temperatures.