The yeast is probably slow. Shaking them will help getting them back in suspension and meet the sugar, which is good.
If you added your priming sugar, as you said, do not add more sugar/carb tablets. It is not the lack of sugar, it's the yeast that needs time. If they are getting carbonated that is a good sign. Just give it more time. What is the temperature of the area you store them in, maybe they are just a tad too cold.