are you using (by any chance) the bottle that come with Royal Canin baby milk??? if so, it's because the bottle is hard (non squeezable) plastic. I hate these bottles. I prefer the ones that you can usually buy loose in pet shops (i.e. they're not sold with milk, they're sold on their own) and they are like the old fashioned Fairy Liquid bottles... you can actually squeeze the milk out of them (although of course you should never squeeze milk into a kittens mouth). The Royal Canin bottles (or any of those hard plastic bottles) depend on the kittens having a well developed sucking reflex, and wanting to drink from a bottle.