A lot of things affect the carbonation. Body of the beer, priming agent, how its administered, how much residual body the yeast will get through over extended periods, what yeast strain was used etc. In this case, I think its a combination of things. Firstly, white sugar is the antichrist. Noone will ever convince me otherwise. If you want to use a simple sugar, use dextrose or glucose. Those priming spoons are not the most consistant things in the world. I would say definately move on to bulk priming. The body is a factor, too. The more simple sugars in the initial brew, the lighter the body and the harder it is for co2 to remain in suspension. The carbonation is more vigorous in the glass, but it also makes the head dissipate quicker. Conversely, if there are longer chain fermentables that continue to attenuate very slowly after bottling, you also can get overcarbonation as well.
It can all get pretty hard, and the results with factory made ingredients can, in some cases, be eratic, purely cos you don't
know precisely what the malts consist of, how it was mashed, etc. Without beating the AG drum, you can't really tell how it's all going to react unless you use the same stuff a lot (as in yeast strain, brands of malt, etc) where you can get a 'feel' for how the lot will react together, and achieve consistant results. Unlike grain where you consider the mash temp, percentages of dextrins, etc, and this is all known to you. This is why when I brewed extract, I always used the same ingredients brew to brew, as in if making a mild I need this brand and type of extract with this yeast, for a lager I need this brand and type...etc.
I think what you said about the body being a factor is true as the couple beers I had that were really gassy, were very light in body and were brewed with the Brew Enhancers etc. Where the ones ive brewed with malt, spec grains and all extract have been completely different and much better than the others.
Also, i think after a period of time the yeast has consumed the simple sugars in the bottle hence the reason I feel they are much better to drink now than at the start.. <_<
nb. whilst more expensive, malt gives a much better prime for kit and extract beer. (with ag I just used dex, cos I had full controll over the body issues, anyway). I used to use a combo of 50/50 wheat malt and ldm. Good, fine carbonation, with a great head.