Well it all comes down to a loss of movement. The travel from the pedal is not getting transfered to where it needs to be, pulling the pressure plate away from the clutch plate.
All the above suggestions by everyone have given you a list of area's where you could be losing this movement.
go through them one by one and eliminate the possibilities.

Pedal box, grab the pedal by hand and see if you can move it side to side or around, if its sturdy your ok.

Firewall, get some one to push the clutch in and out and see if/how much it is moveing, also check visually for cracks and damage.

Check the rubber on the outcable at the bell housing, if you have a h/d clutch it still could be crushing even if it is a new cable.

Also check the dampener rubber at the end of the cable where it attaches to the thrust plate, its a long shot because usually after they colapse you just take up the slack in the cable and all is good, but you never know the roll pin could be pulling right into the steel.

See if the thrust plate is getting surficiant movement.

If all the above checks out ok, then i would say it will either be some of the fingers (springs) in the diapham(on the pressure plate) have colapsed, or the pull back springs have gone soft. This will mean its time for a new pressure plate.

Best of luck.