the computer locks instead of hibernating

Hello!

When I select hibernate from the shut down menu, the screen turns dark and
then a message appears: "Locking computer...". After that the login-screen
pops up.

Hibernate used to work quite well in the beginning, and I have no idea what
went wrong.

Can anyone help me to solve this issue?

I am thankful for any help,
Rui Nunes
.