thread fouling, which can happen if you twist the spool while the nozzle is down on the LP or the machine top plate
Without O-rings on the spindle, the spool does turn freely and its easy for the thread to ravel off it or loop under the spool or otherwise foul. I think thats true however the thread is advanced. An index mark with a felt pen on one of the spool's spokes makes for an easy way to gauge the amount of thread played out. Turning the spool with the vacuum on, I get 8 passes out of one revolution of the spool. You could probably get more. Never thought to pull the thread at the nozzle with my fingers - if you choose that method, then yes, you need the spool to turn freely.
I have no idea if the PRC-3 offers improved cleaning with bidirectional rotation and a three position motor toggle. Sometimes I'll put the nozzle on the other side of the record so it runs outside in and I have no idea if that makes a difference vs inside out.
Tim