Hi Biggles, i have a similar problem to yours as well....when trying to create a turn i am usually off balanced and with the lack of speed i bog a rail and the ride is over.
So speed is everything with most of the turns done on a wave (i hope i am correct in saying that?). As far as a bottom turns goes, i find that it all depends on the wave size and power, heading straight down will generate more speed but if you run too far down and into the flats you may not have enough speed to go back up the face (once again depending on wave size and i usually surf anything between 2 - 5 foot waves). As Jaffa said, try turning mid wave or higher, small turns first then with the increase in speed you will be able to head further down the wave and maintain the speed through your bottom turns.
I am still trying to wrap my head around it myself though but logically that sounds the way to go. Let us know how things go and if you come right then perhaps drop a few pointers for the others who may be struggling as well. Stay stoked! (love that saying)