If you prime (add sugar) per bottle I would advise you change and batch prime as its easier and more consistent, method detailed below.
All I do to ensure a consistent carbonation level is use the calculator found here to determine how much sugar to add (corn sugar is dirt cheap) and then do as follows:
- Weigh out the required amount of sugar into a pan
- Add enough water to just dissolve the sugar and boil for a couple of minutes.
- Cool sugar solution to room temperature and add to a sanitised bucket
- Syphon my fermented beer on top of the sugar solution and gently stir to ensure good distribution of sugar
- Bottle using whatever method you like and leave for 2 weeks at room temp
One thing to watch out for is to ensure you beer has finished fermenting before you bottle. A typical rule of thumb is to wait for 3 consecutive hydrometer readings taken at least a day apart.
Hopefully if you've followed a good sanitisation procedure your inconsistent carbonation levels are NOT due to infection. This can sometimes show as a brown ring around the neck of the bottle and an off smell or flavour.
p.s. I found that the carbonation drops are expensive, terribly inconsistent and had other 'stuff' in so this may actually be the root cause of your problem.