Hi OP
Imo it's a complicated business and it could be any of a number of factors. As PPs have mentioned, if you are feeling at all tense or stressed then that can stop you dilating as your body triggers its fight-or-flight response so that you can deal with the perceived threat as effectively as possible. This was very useful in ye olden days when you might encounter a bear as you were labouring outside your cave, slightly less helpful now. Unfortunately it also works with perceived threats or worries as your body reacts physically to what you're thinking (just like thinking about cake makes you salivate), so feeling nervous can stop you dilating.
Also as mentioned above it could be physiological - you need a good connection between your baby's head and your cervix to stimulate oxytocin production and fuel labour, so if your baby isn't in a good position then this can have an effect.
Or some women just dilate more slowly than others, it doesn't mean it's "wrong" except when you're having to keep to someone else's deadline.
I would also strongly recommend Ina May and also Birthing from Within, which is a bit hippyish but has some good exercises to do to get to the bottom of any anxieties you might have (which might be things like money, how the baby will affect your relationship, etc, as well as about birth specifically). I also recommend Lazy Daisy antenatal classes if you have any near you - I did them when I was pregnant with DS after a not-great birth with DD and loved them - they cover both the physical side of things and work on fear release as well through relaxation/hypnosis. In the interests of transparency I should probably add that I loved them so much and had such a nice second time birth that I trained as a teacher myself :)
Would also agree with PP that a good second birth can really help you come to terms with the first, but talking through your notes with someone is also a good idea.
Sorry if this is a bit long/garbled - have been arguing with DD about how many noses I have while typing
which doens't do much for a coherent train of thought!