Where do you think you might be going wrong? It can take a fair bit of practise to get it right, and of course it depends on the horse too (young, old, unbacked etc?).
There's a good few vids on You Tube you might learn from but it's hard to advise without actually seeing you working with your horse.
I second having a lesson, I long reined with my youngster and had some lessons (with him) and it was brilliant! He really enjoyed it and we both learnt so much. If you can find someone to have lessons with, go for it!
Two most likely causes of horse 'napping' on long reins:
When ground driving you need to allow for the moment of being left behind as the horse goes forwards just before you do. If you don't give sufficiently with the rein at this critical moment, the horse walks into the bit, and will immediately stop. This will very quickly teach a green horse to expect pain in the mouth as he moves off, and he will learn to 'jib'.
Driving, whether from the ground or from a vehicle, depends upon your ability to 'send on' the horse. You need to teach voice commands for walk and trot, which can be done in hand, on a longe or as part of the long lining process. If your horse won't go forward, and you don't have the skills or experience to 'drive' him forward on the long lines, you should go back to in-hand or longe work until the forward voice aids are well established.