Z axis won't initialise

Initialisation fails because of a z-axis problem. It reaches the top - and before you ask, yes it hits the screw.

But the sensor doesn’t register the hit. I have diagnosed this to be because the belt is slightly slack, and the hit is not transferred properly to the stepper.

So here’s the question. How do I put some more tension into the z-axis belt? I’ve tried slackening screws and pulling while I re-tighten, but that only works temporarily.

You insert some washers between the axis cap (a grey injection molded piece holding the pulley) and the aluminum extrusion.