I also cannot access the GRUB menu by any manner of pressing shift or esc on my Asus X205TA. To get the GRUB menu to display at boot, I had to modify the
/etc/default/grub file. See this page.
To get the GRUB menu to display every time (until I change it back) I used nano to edit the file in the terminal. Type:
sudo nano /etc/default/grub
find the line that says
# at the start of this line to comment it out
and make sure
GRUB_TIMEOUT=10 or some other number bigger than zero. When done exit nano saving changes and run