If the steer tube is even with the top of the stem clamp you may not be able to adjust preload. The top cap has to push the stem down on the steer tube to preload the bearings. Once the top of the steer tube gets up far enough, you can no longer tighten the headset. If your top cap has a recess that fits down inside the top of the stem as many do, you cannot tighten the headset even if the steer tube does not quite reach the top of the stem. It is entirely posible that you do actually need a spacer above your stem
A picture of your top cap and of your stem/steer tube could serve to clarify this