From my experience, dystocia, or failure to progress during labor is becoming more and more common.
A recent Danish study was released that attempts to answer why dystocia occurs. You can read the study here. If you're not into reading all the medical jargon, here's my summary:
From this study, women who on admission to the delivery ward that have not dilated past 4 cm, and have a baby that has not descended far enough to thin and spread the cervix were more likely to be diagnosed with dystocia. There also was some evidence that a large baby played some part in dystocia, but it was minor. (Personal side note: Since our bodies are all different, large for one women, could be small to the next. So, not only is it difficult for this study to determine whether or not a large baby could cause dystocia, but it's also very difficult to estimate a baby's birth weight. Therefore, assuming that a women is not capable of a "normal" vaginal birth b/c of baby's weight is nearly impossible.) Finally, the use of epidurals was observed to be associated with the diagnosis of dystocia.
Lack of descent had a strong association with dystocia along with poor head-to-cervix contact, and often led to c-section.
"Descent of fetal head is correlated to dilatation of the cervix, and cervix dilatation <>Epidural analgesia had the strongest association with dystocia among the risk indicators assessed. In total 71.2% of women who were treated with epidural analgesia were diagnosed with dystocia. A similarly strong association between dystocia and epidural analgesia was reported from a population-based study of 106,755 deliveries without induction and with durations of delivery <>Alehagen et al. found that women who received epidural analgesia had experienced more fear, but not more pain, before the administration of epidural analgesia than did women who did not receive epidural analgesia  and fear may prolong duration of labour . Recent reviews come to the conclusion that epidural analgesia appears to prolong labour's second stage and prompt more use of oxytocin [25-27]. Although we excluded from the analyses those who were treated with epidural analgesia after being diagnosed with dystocia, reverse causation is still a possible explanation of the association we find. If a need for pain relief or fear of pain are among the clinical precursors of dystocia, epidural analgesia could be part of the mechanism leading to dystocia." [Emphasis added.]
The conclusion of this study is that it provides evidence of an increased risk of dystocia for women who, at admission to the hospital, have a cervix dilation < 4 cm, the cervix is tense with a thick lower segement, and that there has been poor contact between the fetal head and cervix. This could even be a significant predictor of dystocia, but more studies should be done. It is also clear from these observations that there is a great association between the use of epidural analgesia and the increased risk of dystocia, and it could have a causal explanation.