In a nutshell.
All those plates and discs will expand from the added heat and pretty much eats up any movement the Barnett pressure plate needs to disengage all of it.
To get the mechanical advantage through the clutch handle, the lever at the clutch cover pulling on that small gear that pulls on pressure plate you're really not moving much in there to get the plates to separate a small bit.
Loose that clearance and a clutch will drag, badly.
Double check your cable free play. Too much will amplify the problem.