How do I show the GRUB menu in GRUB 2 during boot up?
If you want to enable the menu by default, check the line
GRUB_HIDDEN_TIMEOUT in the file
/etc/default/grub. This option does the following:
Wait this many seconds for a key to be pressed before displaying the menu. If no key is pressed during that time, display the menu for the number of seconds specified in GRUB_TIMEOUT before booting the default entry. We expect that most people who use GRUB_HIDDEN_TIMEOUT will want to have GRUB_TIMEOUT set to `0' so that the menu is not displayed at all unless a key is pressed. Unset by default.
As you see it is unset by default, so check whether it has been set or not in your case.