Ok...
An actuator is very simply, a shaft, with a cap at one end, rivetted to rubber type diaphragm. Underneath the diaghragm is a spring.

The boost acts on teh rubber diaghragm and compresses the spring, thus sending the shaft outwards, which then operates the wastegate it is attached too as required.

When the spring softens due to simple metal fatigue, it takes less boost to move it and thus the wastegate is operated too soon, thus the loss of boost pressure.

And it really is as simple as that.