sudo --list to show us the active sudo configuration for your account:
skath@beast:~$ sudo --list
Matching Defaults entries for skath on beast:
env_reset, mail_badpass, secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin
User skath may run the following commands on beast:
(ALL : ALL) ALL
Check the timestamps on the cached credentials, this could help get an idea why you're not being prompted:
skath@beast:~$ sudo ls -al /var/lib/sudo/$USER/
drwx------ 2 root skath 4096 Jan 28 14:27 .
drwx------ 3 root root 4096 Jan 28 13:52 ..
-rw------- 1 root skath 40 Feb 5 16:18 1
-rw------- 1 root skath 40 Jan 28 14:51 2
-rw------- 1 root skath 40 Jan 28 13:56 tty1
sudo --remove-timestamp to try to wipe any cached credentials.
Similar to the -k option, except that it removes the
user's cached credentials entirely and may not be used
in conjunction with a command or other option. This
option does not require a password. Not all security
policies support credential caching.
When used without a command, invalidates the user's
cached credentials. In other words, the next time sudo
is run a password will be required. This option does
not require a password and was added to allow a user to
revoke sudo permissions from a .logout file.
When used in conjunction with a command or an option
that may require a password, this option will cause sudo
to ignore the user's cached credentials. As a result,
sudo will prompt for a password (if one is required by
the security policy) and will not update the user's
Not all security policies support credential caching.