How exactly do you predraft?
Do you just split the roving into strips or do you also pull on the roving a bit to open it up?
Are you drafting any or basically predrafting it right to the goal weight and just letting twist travel into the roving?
What you could try to do is to space your hands a few inches apart from each other (how far depends on the type of wool, so if the roving doesn't seem to budge, position them farther away) on the roving and gently tug on the roving opening up the fibers and also making it thinner in the process.
It sounds like you're trying to predraft straight to the size you're looking for. Maybe you should try to predraft thin but not too thin, say to a pencil's width, and then try to draft to the desired thickness while spinning. It's less labor intensive and once you get the hang of it, you'll be able to spin smoother and thinner.
There's alot of helpful websites out there for spinning, alot of them discussed in other posts.
Of course I must add, don't be afraid of breaking the roving since once you get the hang of it, joining a new roving is no big deal, and you're going to need practice getting the hang of it anyways.
And yes, of course, you'll be able to spin thinner and more consistently as you keep on spinning.