The HUD reflects on both the inner and outer layer of the windshield to give a sharp composite view. Two possible problems in your case:
If the windshield is the wrong one it won't have the correct reflective surfaces, and all you will see is a poor (low intensity) reflection of the display on the HUD emitter in the dash.
If the windshield isn't exactly the same distance from the emitter (this could be because the shape of the glass isn't the same, or even just a thicker layer of adhesive) the display will be out of focus. If this is the case, they can re-focus the emitter to get a sharper display.