The cider may not have been stirred enough at point of flavour addition; this is essential for ensuring sufficient yeast is transfer into the bottle for secondary fermentation (bottle conditioning).
Finings may have been added prior to bottling. Finings are usually included within the kit and should be used for kegging only – use for bottling will prevent carbonation (some homebrewers have used the finings to get a clear cider, which isn’t necessary as it will clean naturally in the bottle over time).
It’s also possible that that the bottles have not been left long enough, sometimes the cider can take 3-4 weeks to fully condition.
