@ David
It sounds like another plugin or possible the theme is not limiting their javascripts to their own admin pages and causing a fatal javascript error preventing the importing javascript from running. The first 100 are triggered via the Import button then javascript is used to batch import 100 at a time via javascript requests. If there are fatal javascript errors, this will prevent the batch import requests from happening.
To see if this is the issue, navigate to the Connections : Import CSV admin page and hit the F12 key. Now click the Console tab.
Import the file following the normal steps.
On the last step, with the progress bar, take note of any javascript errors being displayed? What are they? Often the error message will give clues as to the source of the error.
Hope this helps, let me know.