It sounds like the sugar isn't dissolving or being evenly distributed in your bottling bucket. When bottling, take about a quart or maybe a little less of beer from the secondary and put it in a pan on the stove, heat it up to a gentle boil and add the corn sugar. Stir to dissolve. This process has two benefits, first, you make absolutely sure that your corn sugar solution is sterilized via boiling, second, you make sure that the corn sugar is completely dissolved. Remove the pan from the heat. Now siphon the rest of your beer to your bottling bucket while the pan cools off a little. When all of the beer is in the bottling bucket, slowly pour in the priming solution (avoiding splashing if possible) and stir the entire bottling bucket with a sanitized spoon. Make sure it is well mixed, then bottle as usual. Don't worry about the hot sugar solution, it will only raise the temperature of the whole batch a few degrees.
One other practice I used to do when I was bottling was make sure that I siphoned up some of the yeast from the bottom of the secondary into the bottling bucket. When everything got stirred, I could be sure that there was some yeast present in every bottle.
As others have suggested, make sure your bottles are capping correctly. One thing you can do to check this is to take one or two bottles from the batch and age them upside down. If any leaks are present, beer will leak out around the cap. If there are no leaks and the beer is still flat, either your yeast was dead or there was little or no yeast present at all.
Prosit,