It would be lack of tension if you can't downshift. Could be a cable routing within the shifter problem. Those things can be tricky to get back together properly. Try it again in gear 8. Make sure you have all the slack out of the cable. Try running through the gears without the cable attached to the derailer while gently pulling on the cable. See if the cable pull seems normal.

You can usually replace cable on Sram MRX and some other Gripshifts without disassembly, by the way.