The most common cause for a premium RV day/solar slow rise roller shade not staying locked in the down position is a worn or faulty internal spring mechanism. The locking function relies on this internal spring. To fix this, you can replace the spring with a 1.25" diameter replacement spring designed for premium RV roller shades. For your 26" x 51" shade, confirm the roller tube diameter is 1.25" and then order the appropriate spring size. For the 61.5" wide shade that lacks tension, use the same 1.25" diameter spring but select the LARGE size option (60-70 inches total length) to accommodate the wider shade. Replacing the spring should restore proper tension and locking functionality. You can find these replacement springs on elkhartrvparts.com under the Irvine brand, slow rise version, with size options for your specific roller tube diameter and shade width.