How to troubleshoot CRM Online Data Import Wizard errors

I have a zipfile containing 90 csv files which I need to import into our Dynamics CRM Online. I have already done a lot of cleaning on the files (ensuring that none are over 32MB and the that the whole file is less than 32Mb), but whenever I try to import the zipfile, I get a popup saying that "File isn't valid" with text underneath saying that the file could not be uploaded because it is corrupt or does not contain valid files. I have tried to import some of the the files individually, and they do import - so why am I getting this message? Is there a logfile to check to find out which csv file is failing? Do I have to go thru each file individually to find out which is failing?