I had notice that in SharePoint Online, when I create a content type in the content type hub (http:/my.sharepoint.com/sites/contentTypeHub/), the site collections created before the content type never subscribe to the hub. But if I create a new site collection, after the content type is created, then that site collection show the content type immediately.
So if this is happening to you you should first publish your content type, at your content type hub. From there you should click "Manage publishing for this content type" and you will see 3 options:
Publish Make this content type available for download for all Web Applications (and Site Collections) consuming content types from this location.
Unpublish Make this content type unavailable for download for all Web Applications (and Site Collections) consuming content types from this location. Any copies of this content type being used in other site collections will be unsealed and made into a local content type.
Republish If you have made changes to this content type, the content type needs to be "republished" before the changes are available for download to Web Application consuming content types from this location.
If no date/time is shown at "Last successful published date:" and the Unpublish and Republish options appeared to be hidden/grayed then you have to publish the content type. To do that just click Ok, and the same to the popup. Then you need to go to Site settings > Content type publishing and make sure "Refresh all published content types on next update" is selected and click Ok. After that just wait like 15 minutes.