If you just crank the truss rod nut tighter, there's a good possibility you're just digging the nut itself tighter into the wood. The trick, when you want to reduce the neck relief (i.e., tighter the truss rod) is to help the nut by
gently pulling back on the headstock, and
then snugging the nut. IOW, don't try to get the truss rod to pull the neck flatter; flatten it by hand, and set the nut to that point.
A whole turn is a
lot. Don't keep doing what you're doing!
Hit the repair section of the Bass Guitar Forum and look for stickies on basic setup. I strongly recommend you read up on truss rod adjustments! Those threads have been very helpful to me.
