If you are making major adjustments, the following may help.
The strings must be routed from the center of the spool to the outside thru the two holes. Routing the other way makes it impossible to tighten or loosen the strings except by tying knots.
Use strong spring clamps found at Home Depot or similar for 99 cents to clip to the window frame or the valance to hold the shade up while you work with the strings.
With the strings run thru the spool from the middle and leaving slack in the string, tie a knot in the strings. With the center screw snug but loose enough to turn the spool, turn the spool COUNTER CLOCK WISE, winding the string onto the spool a couple of turns until it tightens up. Tighten the screw. This way, tenson on the string will tend to tighten the screw, making it very unlikely to work loose.
When both sides are done, remove the clamps and work the shade, adjusting the spools for desired curtain tension.
Bigfoot did all of mine wrong, making adjustment impossible without re-routing the string from the inside of the spool.
Charles