Hi Robin,
Based on current situation, I recommend you refer to the link below to troubleshoot it.Search does not return all results in SharePoint Online for Office 365
http://support.microsoft.com/kb/2008449/en-us
Please Note: Uploaded documents may not immediately show up in search results. This may be due to the interval for which the search crawler is set. Once content is crawled it will appear in search results.
Thanks,
Aaron Yuan