i know this might sound silly but - sterility.
have you correctly sterililized your equipment/bottles prior to bottling? also, have you sufficiently rinsed the sterilizer from the bottles?
also, dextrose is awesome for priming
i have a system i use for priming the bottles which i already bounced by oliver. (hey oliver: i've got a cool version of it for ales with different malt powders
very cool.)
what i do to prime my bottles is to add between 180-190g of dextrose (depending on the style of beer - usually i just use 180g) to 500ml of boiled water in a jug and dissolve the dextrose. then what i do is i purchase a 20ml syringe tube from the chemist and prime at a division of 500ml per 22.5L. so 18ml for 800ml and 17ml fir 750ml bottle.
also, if you're brewing a darker style of beer - you can use malt powder
by darker, i mean anything brewed with a lager yeast like lagers and pilsners because a discoloured head on those lighter beers is not formally correct to the style. though, i guess you could make a dark lager of an oktoberfest and prime with light malt powder and it would be awesome... mmm good idea
anyways, to convert to a malt powder priming recipe all you have to do is use malt powder instead of dextrose. you still use 180g per 22.5L of brew. you also should leave the malt primed beers fractionally longer in the bottles before drinking. all i mean is don't go scoffing em when they're younger than 6 weeks cause you really won't see the complete benefits of the malt powder priming. you'll definately notice a lot of difference in the head/body/finish but not completely. malt primed beer has the finest head of any primed beer. in fact, i'm completely confident that an accurate kilkenny clone could be finished close to the real thing by priming the bottles with light malt powder and that an accurate guiness close could be finished by priming the bottles with a dark powdered malt.
if any of you are thinking of experimenting with malt priming, then only prime a few of your bottles with malt and prime the rest with dextrose. the reason i say this is sometimes malt priming just isn't the right thing for the brew even though it just totally seemed like the perfect thing. but dextrose, however, ALWAYS gives great results. plus i, personally, have no problem going cooper's sparkling ale style and mixing my dextrose sediment through my bottles to drink them
awesome.
cheers
-wombat