The two strings are gripped by two rough metal rollers. You can see the strings coming through them if you look up into where the strings come out of the blind body. The rollers sort of slide/fall into place by gravity (at least one of them does) and grips the strings between them with friction. Take the blind down so you can inspect that the rollers can freely slide back and forth and that nothing is obstructing them. This is done by sliding off the fronts of the plastic brackets. If your brackets are metal then the fronts probably fold away. If you don't find any obvious problems then it might be that the string(s) has been worn with use and is now too thin to be properly held between the rollers with friction. This eventually happens to most strings if the blinds last long enough. I would guess that these blinds are old and the strings wore out (thin). It usually isn't worth trying to find the right string and it is tedious to restring them. If you really like the blinds and they are expensive, then it is possible. The strings go through the rollers, then through a pully on each side, and then through slots in each louvre and attach to the bottom rail of the shade. Basically, when you pull up the shade, you just pull up the bottom rail and everything else comes up with it.