If you have added approximately the right amount of priming sugar and your beer is not carbonated at all, your problem probably is not the amount of sugar added.
A common problem is inadequate mixing of the priming sugar in the batch, but this doesn't apply here because you indicate that you added sugar to each bottle individually.
Other potential problems (though unlikely assuming basic understanding of the process) is lack of viable yeast in the bottles, insufficient time for the yeast to consume the priming sugar, or storing the bottled beer at too low of a temperature for the yeast to become active and consume the added sugar.
Your problem is more likely a failure of the caps to hold pressure and thus allow all the carbonation that forms to escape.
If you added approximately enough priming sugar in the first attempts and got no carbonation at all, then adding more and not changing anything else is unlikely to solve the problem.
If the problem is temperature or time, store the bottles at room temperature for a week or two. That should solve it.
If the problem is lack of viable yeast: open each bottle, add a tiny amount of dry yeast or a drop of liquid yeast slurry, and re-cap. Store the bottles at room temperature for a week or two. In this case, do not add more sugar (because the sugar you added before is still there).
If the problem is bad capping: open each bottle, re-prime, and cap. Store the bottles at room temperature for a week or two.
I have had a couple batches that I over-primed a little bit. The observed behavior was that about three seconds after removing the cap on a 22 ounce bottle, about eight ounces of foam would emerge from the bottle. Foam would continue to form inside the bottle for a few minutes (and if I had only poured out a few ounces, enough foam formed to emerge from the bottle).
This was not catastrophic, but would create a mess if I wasn't ready to pour immediately into a glass.
I suppose that the more you over-prime, the faster/more violent this process will be. If you prime enough (or, more likely, bottle before fermentation is complete), you can create bottle bombs (the bottles can explode), which is dangerous and very, very messy.