Even though the lip is visible through the mouthpiece, the alignment may still not be correct. The air stream should hit the lip straight on and not at an angle. To check this, insert the slot stick into the mouthpiece with its thin end (without the masking tape) extending into the voicing opening. The slot stick should be parallel to the top edge of the voicing's side walls. (indicated by the arrows)

If the slot stick slants down, the mouthpiece is sitting too high.(Figure 1) Remove the mouthpiece and cut out clay from the opening. (Figure 2) Reposition the mouthpiece and re-check the alignment.

If the slot stick angles up (Figure 3) the mouthpiece is too low. Remove the mouthpiece and attach a coil of clay with slip. (Figure 4) Reposition the mouthpiece and check the alignment.

Mouthpiece alignment diagram.