1. check there is enough spacers under the top cap so any lips are not bottoming out on the top of the steerer.
2. check the star nut is low enough in the steerer so that the base of the top cap isn't bottoming out if it has a stepped bottom.
3. if the steerer is tapered make sure that the taper finishes well before the split taper ring that sits down into the top bearing as if this sits on top of the taper and not into the bearing it will prevent the headset from tightening or cause it to losen (depending on how close it is).
4. Make sure all the bits of the headset are there.
One way to check if it's a tapered headset is look at the crown on the fork. It will be bulkier to accommodate the 1.5 inch section of steerer and looking underneath you should see the steerer pass through the bottom.