If the hose clams are snug then it is most likely a compression issue or crud between the carbon and stainless faces..
There are two set screw holes in the stainless rotor located at roughly 10:00 & 2:00. There should be two set screws in each hole, one on top of the other.
Remove the set screws and back the rotor off until it is just touching the carbon face but there is no compression in the bellows. Mark your shaft with electrical tape, at the forward edge of the stainless rotor, then slide the rotor towards the stern using the compression distance, for your dia, shaft from the PYI instructions.
If you don't REPLACE the set screws there is no guarantee they will bite the shaft as the volcano head gets rolled or flattened!
These are new:
You then REPLACE the set screws with new and re-tighten them at the newly measured compression distance from the tape you placed.
This is a photo where you can see the set screw holes in the stainless rotor..
I then place a 316 SS Ruland split collar behind the stainless rotor as added insurance to prevent the rotor from sliding forward. These are available from McMaster-Carr.
Ruland Split Collar installed on a v-drive boat. Note the white tape where the compression measurement was taken off.
P.S. could just be as simple as some junk caught between the face seals..