Any RV mobile repair will restring them for you for around $65.00.
OR
Go to any camping store or RC dealership that has a supply store. There is a real nice restringing kit available now. Comes in a couple different colors to match your shades. Includes the string, new mounting screws and instructions. Waay cheaper than ordering it on line.
If they are tight or not working correctly the next thing that WILL happen is the string breaks and now you will be left with a flopping shade all over the place! Restring them before that happens!