I have a Shimano RevoShift 7 speed (indexed), that has trouble shifting between 6 and 7th gear. I disconnected the cable for a test, and I see that for the other gears, each click moves the cable about 2mm. But between 6 and 7, the cable only moves about 0.5mm. Is there a way to fix this? I do hear clicks for all gears.
Revoshift and gripshift have to be the cheapest, nastiest, most accountancy-driven parts on any bike. They don't appear on mid-range or higher, only on bottom-entry level bikes.
SRAM and Rohloff twist shifts are much better, so its not a concept problem, its a design and materials problem.
Your best option is to buy a 7 speed pod shifter, which might also be called a thumb shifter, or even fit an old-school friction shifter with enough range.
A Shimano Altius M310 shifter as pictured should work perfectly for you, and its really cheap.
Do consider replacing the gear cables inner and outers at the same time, because you may-as-well.