I've heard rumors and read several books that have stated that epidurals can stall the progress of labor. Here is a Danish study which has found that to be true. The study included 2810 women each expecting a single baby (no twins).
"Vaginal examinations at admission provide useful information on risk indicators for dystocia. The strongest risk indicator was use of epidural analgesia and if part of that is causal, it is of concern."
Some characteristics they found related to labor dystocia (slow or stalled labor) were poor cervical presentation, including dilation less than 4 cm, tense cervix, thick lower segment, poor positioning of the baby (baby's head above inter-spinal diameter, poor fetal head to cervix contact), and the use of epidural analgesia.
I would like to point out that all of the above characteristics (with the exception of the epidural) are generally indications that the baby is simply not ready to come out. If the baby is in a poor position, such as high in the birth canal, or the cervix is not dilated or thinned, it's a strong hint that the baby is just not coming out any time soon. There are things that can be done to correct poor baby positioning (see my post about Posterior Positions), as well as things that can be done to help thin the cervix and augment labor naturally. If these do not yield more progress, then it may just be a good idea to wait and let your body and the baby prepare more and get ready for birth. Contrary to some current obstetric beliefs, there is no set time table for labor. Some women labor for just a few hours, while others (myself included) may labor for a few days or more. Women's bodies are made to birth babies and most of the time can do that in their own time unaided by medical intervention.
There are times that intervention is needed. For instance, when the bag of waters has ruptured there is a limited time that the baby needs to be born in order to avoid infection, but that is generally 24-36 hours and can still give a woman's body time to work and birth the baby. There are other instances, but I believe in most cases intervention is not needed, and in the case of epidurals, intervention actually can stall labor and make things more difficult.