I am a retired high school teacher with a passion for dachshunds. My mini-dachshund is Steffi, she is 18 months old and we have been attending Obedience classes for about a year now. I hope to be ready to trial in Rally O in 2015. The main hold up is my handling skills

Steffi does very well on most things but I am sure I must frustrate her at times. She is now in the highest class offered in our school so she is doing a bit of agility, so here she is doing her first jumps over a pool noodle (she has since moved on a bit and is now a confident jumper). Steffi started out very fearful of other dogs and people, since going to the school she is a lot calmer and has some favourite dogs.
One problem we are having is every now and then she just stops - ie we might be just approaching the last cone in the serpentine and she will just stop. I have no idea what it is I am doing wrong - I have been making sure I am not crossing into her path and I am looking at the cone ahead (rather than the one we are at) but we still have the occasional halt.
Welcome to DTA. It is great to have you here. There is another dachshund on here somewhere who does RallyO so do look out for him.
The first things I think of with stopping are pain, uncertainty and fear. I'm sure there are lots of other possibilities and you've probably already thought of these but they are the one's I see in my dog.
Pain? I have a dog that used to stop suddenly when running, sit or lie down and look worried, it turned out he did this when he had a really bad gut pain. Could it be that doing the serpentine cones is causing Steffi some kind of pain maybe in her back and then after a short break she is able to continue? It could even be something to do with the level of enthusiasm she has for reaching the end of the cones that means she triggers something always near the last cone. Watch how she is moving, is there a particular movement (a head turn or a bounce for instance) that might trigger it, is she touch sensitive anywhere? Can she get up instantly and run on happily or does she NEED a pause?
Uncertainty? If Steffi does not like to get things wrong (and which of us does?) she might stop before the end of the serpentine while she still has something that she knows how to finish and waits to have you show her where to go next (which you do incidentally by luring) so she doesn't have to face the uncertainty of reaching the end of the cones and not being sure where to go next. I'm not sure how clearly I've explained that

. You'd need to see whether she ever stops when she knows exactly where to go next and whether she often stops when you don't tell her where to go next. If it is this you could set up a serpentine which leads on to something with great certainty and then improve your signals and Steffi's understanding as you start adding in a little more complexity as Steffi gains in confidence and understanding.
Fear? See if there are any linking environmental factors however obscure - light level, surface, a dog barking, smells... dogs are very sensitive and sometimes there are linking factors which we don't notice which link back to a memory we didn't know they had - for instance the first incident could have been triggered by a pain which you couldn't know about but there is something that reminds her. This is particularly possible as she was fearful. Has anything ever startled her as she was finishing the serpentine? Maybe she just needs a moment to check out the environment and make sure nothing scary is going to happen. If it is this you need to show her that those environmental stimuli are not scary anymore.
Hope some of these might give you some ideas anyway! Please let us know how you get on, I'm fascinated to know what the cause is and how you solve it.