Can't go any further as you are running out of thread or they are getting too much tension? Either shorter driveside so they can tighten more or something else is wrong. Longer spokes would just run into the same problem a few turns later unless they are actually to short to engage enough.
From an equalish tension all round, say where the thread just disappears into the nipples add a few turns to all the spokes until things tighten up a bit, then add more to the driveside until dish is right, then sort out any wobbles, adding more until tension is enough.
Thanks snottyotter and cycleclinic - I've built it up now and it's fine. I was a bit worried at how highly tensioned the drive-side needed to be at first, and thought something was wrong. Those nipples really took a lot of twisting.
Can't go any further as you are running out of thread or they are getting too much tension? Either shorter driveside so they can tighten more or something else is wrong. Longer spokes would just run into the same problem a few turns later unless they are actually to short to engage enough.
From an equalish tension all round, say where the thread just disappears into the nipples add a few turns to all the spokes until things tighten up a bit, then add more to the driveside until dish is right, then sort out any wobbles, adding more until tension is enough.