On the seitz blind you will need to remove the whole blind from the wall and remove the blind and roller from the housing. If you turn the shaft clockwise then the spring should tighten. if the spring does not tighten then it it likely that you will need to replace the spring, our ref. 135876.
When you have tightened the spring you need to fit the tensioning bush, our ref. 124190, and replace it in the housing without losing the tension (this is fun). In future you should be able to tighten the spring, if necessary, by using a flat blade screwdriver inserted into the tension bush without removing the blind from the housing.