Yes the default import process is not great in terms of error messaging. I use this all the time https://apps.shopify.com/excel-export-import worth the money (you can cancel it once you've used it so only pay for one month) for all the time it saves you. It gives much more granular error messages per line to tell you exactly what the issues are.
This worked, thank you.
However it only imported 1633 out of 6000. It's been a good few days now and it hasn't budged. I have tried to import again but it won't let me. It says there is already an import in progress.
It doesn't look like it is importing any more.
Can you help?
6000 customers should only take about 2hrs max (usually a lot quicker) so something wrong there. I would cancel the import and then try again (not sure how you cancel one though as never had it happen to me before, but start by logging completely out, closing browser and logging in again)
Hmm - unfortunately I think the only way to clear that is to contact Shopify Support and get them to do it. https://help.shopify.com/en/questions#/contact
You should see this on that page I gave you:
But their email is 'email@example.com' or maybe try Twitter support team https://twitter.com/ShopifySupport