If yours are like mine, the string that is secured at the bottom of the window will tighten the tension. Loosen the screw that holds the little round wheel and turn the wheel to tighten the string. Also, make sure that the string is wound so that it is keeping the retaining screw tightened. I had several that were wound counterclockwise and kept loosening the screw so the string would get slack and the blinds would fall.
Hope this helps,
John