Is your new steerer longer than the original? Look under the top cap, if there's no gap between the end of the steerer tube and the top of the stem (or the top of any spacers on top of the stem) then no matter what you do with the top bolt, you won't be able to tighten the headset.

Alternatively, if you've replaced the steerer I presume you took the headset to bits. Are you sure you didn't leave out any parts while assembling the headset? As in, is the crown race still on the old steerer or something?