error in primary group changing of debian

There is terrible problem with primary group changing in debian. When I unleashed this code

sudo usermod -g primgroup user

the shell doesn't respond. It looks like that it hangs. But when I tried on other shell, it says that it change nothing (so apparently the first command [the hang one worked]). Still it is a mistery! Furthermore when I tried to create a file, and check it using

 ls -l 

it still gave me the old primary group. Amazingly after I restart my computer, tadaaa, everything set up as expected. So? I don't know