The most common is that the relevant entry has not been published, or that some connection referred to in the entry has not been published. This could be, for example, a link in a patch. There is also the possibility that sync services are incorrectly configured if the change is within the patch or content quoted on the page. Try making a small change on the page itself (for example, adding a space or point in the introductory text) and save. If it works, you need to get a programmer to change the sync service settings.