Both Habs and Edward176 have made very pertinent comments and given some food for thought. I wondered just how big the commercial freezer you use as a smoke house is and does this contribute to the poor smoke density if it is too large?
You also mentioned an AC unit, is the smoke house mechanically vented or does it vent naturally? I.e could it be sucking the smoke out too quickly? Could the vent be too big / effective? Forgive me if I have misinterpreted your original post re the vent?
Taking in to account the other posts could you have created a perfect storm of contributory factors including use of a propane burner affecting oxygen levels that is simply too much for the bisquette burner to cope with?
The puck burn time of 20 minutes is designed to slightly under burn them so as to produce a clean smoke which Bradley highlight in their sales blurb but in your case this does seem excessive.
At least you now have a few more things to think about and maybe work through!?
Welcome to the forum.
