Ctrl + Space not working for content assist on eclipse

My problem is that the content assist window won't pop up after clicking Ctrl + Space. I checked my shortcuts and the content assist is bound to Ctrl + Space. Sometimes the window automatically pops up making suggestion after typing dot e.g., but this won't happen when using the keys combination.

Eclipse version:

Eclipse Java EE IDE for Web Developers.

Version: Kepler Service Release 1 Build id: 20130919-0819

My OS is Ubuntu 12.04 if that matters.

In my case; Window>Preferences>Java>Editor>Content Assist>Advanced

Make sure Select the proposal>Template Proposals,SWT Template Proposals,Java Proposals ,Content assist cycling>Template Proposals,SWT Template Proposals,Java Proposals is ticked.

I managed to fix it finally. The problem was that the ibus was overriding the shortcut so I deleted this shortcut configuration.

Type ibus in the dash:

Then select Keyboard Input Methods. From there click on the first three dots next to the Enable or disable textfield. This windows should appear:

If there is something bound to Ctrl + Space simply delete it.

Go to Windows > Preferences > Java > Editor > Content Assist > Advanced > select all Java only. If you select all, then multiple occurrence of same instance would be shown in Suggestion list. which might be annoying some times.

For that Go to

Eclipse --> Windows --> Preferences --> Java --> Editor --> Content Assist --> Advanced --> select all Java & Java Type Proposals.

and check mark : Java Proposals(Code Recommenders)