After drawing a standard rectangle, changing the roundness property has no effect. In fact the value gets cleared out when unselecting/reselecting the rectangle. When I use the up/right arrow while drawing the rectangle, the corners bevome rounded, but changing the value yields the same results...no change and it gets reset to the initial value.
Can anyone help?
Sounds peculiar. Is it a new thing? It sounds like a glitch with the Properties panel. Have you tried quitting and restarting the application? (Or maybe even rebooting your computer?)
As a backup, remember that there's a Rounded Rectangle smart shape available in the vector shapes menu. It offers a few additional corner options not available on a standard rectangle.