If I am understanding you correctly, you have to use the proper spacers above the steerer to create the right amount of recess above your stem for the top cap to draw the stem tight into the steerer when you tighten the top allen screw. The top cap thickness has to marginally exceed the amount the fork is recessed below the top of the stem. This is accomplished with proper spacing using spacers. Then tighten the stem to the fork after you have the head set bearing pre-load adjusted by snugging the top allen bolt. Posting a pic or two is very helpful for solid feedback.
HTH,
George