I have seen this problem many times, and it is usually dust and dirt built up in side the pressure plate. The bell housing is not sealed and allows dust in . Try removing the inspection plate and the crank sensor, then use compressed air to blow out the bell housing and clutch threw the crank sensor hole. If that doesn't work, you well probably have to remove the plate to clean it, and in my experience you are best off to replace it. The diaphragm spring usually starts to flex in the wrong spot and does not disengage the clutch disc.