A: The priming sugar has been missed or incorrectly measured. It is quite easy to miss putting sugar in a bottle during the bottling process.
B: The cap has not been clamped on properly or sufficiently, this is a common problem when using hand cappers.
C: You put the bottles straight into the fridge after bottling, instead of leaving them out for the first 2-3 weeks to secondary carbonate.
It is quite normal to feel a bit wary about hitting the bottle too hard when using a hand capper, and therefore leaving the cap a little loose. If you have been persisting with a hand capper, do yourself a favour and buy yourself an auto capper and I guarantee you won’t look back!
Twist top bottles can also present a problem because they tend to jam into the bell of the capper and are twisted out. Under such circumstances they should be tilted out and not twisted out as this can lead to the cap being loosened.