One other part that is important for these to work correctly. The top of the piston has to be able to bleed out air. You will notice that the cap is pressed into the piston. The cap can be pressed out. It has threads for air to escape and a small vent at the top (circled in red).
If the threads have filled with gunk over time there will be no way for the air to blead out the top and the tensioner and it will be soft. You can knock the cap out from the inside, clean the threads and press it back into the piston.
john