Because ServiceNow holds processing requests (without a response) until processing is complete, it can appear that an import is failing. You might even see an error message because the Skyhigh CASB proxy doesn't get the proper response from ServiceNow. This doesn’t mean that the import has failed.
You can work around this by importing data using ServiceNow Import Sets. Find more information about supported file types and other important considerations here: http://wiki.servicenow.com/index.php?title=Importing_Data_Using_Import_Sets