2 things I can come up with as I have these shifters is:
1. Your outers/inners are stuffed and need replacing – shifting down require the rear mech return spring to overcome the friction in the cable system. If this is too great it wont shift properly.
2. The cables inside the shifters have become bent and foul inside the shifters.
3. Shifter springs have gone.
4. Shifter mech is sticky/fouled up as about GT85 soaking 🙂
Shifters are poss servicable if anything inside is broke like springs 🙂 Also a friends bike had some 8 spd sram shifters and would click shift down but cable was too grotty and the rear mech wouldn't shift.
Edit – sounds like you have new cables oops – have you replaced the outers though?