Try tightening cords equally at the bottom, not too tight.
If shades need re-stringing, you may want to take them to a local blind shop and have it done if you lack patience, etc. Get the correct re-stringing kit for your shade, i.e. number of strings and look over routing of the strings etc. as you disassemble them. A stiff piece of wire, masking tape, ingenuity and patience are needed. Follow cord routing directions very carefully. Work on a bed or large table. Expect to spend an hour or so with some fiddling and profanity on the fist one