"Groups" tab missing when creating content

Hi all,

When creating content I normally get a tab with "groups" (between menu settings/book outline/etc.) and I can select a group to place the content in. Now the tab is missing and I can't get it back, whatever I try. I have setup a new site to test the feature and all is working fine but the production site I cannot get it back and so nobody can place content in groups no more. I have uninstalled en installed the OG modules, tried almost every setting possible (as far a I know) so I ran out of options then to start all over.... This is my last option.

Is this a bug or am I doing something wrong? Hope to hear from you soon,