Ubuntu desktop computer doesn't seem to lock correctly.

Ubuntu desktop computer doesn't seem to lock correctly if I click "Lock Screen" it does lock if I use shortcut. If I use the "lock screen" button then anyone can unlock it by merely pressing "ctrl+alt+f7".
The problem is not that I cannot lock my screen, the problem is I was under the impression that my computer was being locked when it was not.
now that I know that the "Lock Screen" button doesn't work correctly I know of other ways to lock my computer still the lock screen button should work and it looks like it locked my screen,but if I press "ctrl+alt+f7" then it magically unlocks

So it is a security incident.

Thanks in Advance.