Am I doing something wrong?
It should work as I describe, so maybe?!
Have you got guide lines turned on? These are useful to display as they will appear at the point the wall will stretch to, straight after pressing the plus/minus, so you don't actually have to press return to see the result.
So to make a wall 0.5m shorter for example, start to stretch the wall but keep the cursor at the end point, then type 0.5- in the tracker. (Or the equivalent in feet/inches). You will see guide lines appear at the destination. Hit return to confirm.
What are you typing?
Someone did an nice animated gif in an older thread that describes the process. I'll have a look for it.