These things work on a combination of tension and friction. Examine where the strings exit the bottom of the shades and if the shade is wide, there may be one or more places where the strings loop across and go back up into the shade. If it appears the strings have cut into the guides, you will likely be considering replacing the shade in the future. If you are asking about your '99 model, as I recall they used something that looks like a plastic sewing machine bobbin at the left and right bottom ends of the shade to anchor the strings. Determine if one end is not as tight as the other and either give the slack end an extra wrap around the bobbin, or untie the knot, add tension and retie the knot. The anchor points may be different, but the solution is to add enough tension to the string and try to get the left and right about the same. Don't overdo it. Over time, these strings can break and if the shade is otherwise in good shape, it is possible to restring it but the first one you do can be a real challenge.