If it's the type I've got sounds like the tension rod is sticking in the tube give it a small squirt of WD40 ensure it moves in and out of the blind tube easy,springing out under spring pressure.
I may be your pushing the rod in using the black tension collar and it's not springing back and pushing the black tension collar back to its parked or stop position in the end of the casing, hence it slips and tension is lost.
Charlie, August 2010
