I know there are priming sugar calculators out there to help decide how much sugar one should add but is there any thing as too much priming sugar and if yes what would be the side effect to expect?

I have brewed two batches now but they both lacked carbonation and were out right flat. And I was using Northern Brewer's Fizz drops for priming but the complication was I have 16oz bottles and fizz drop were for 12oz bottles, so I added 1 and a half drop in each of the bottle. I am thinking of adding more than that for the next batch.