Customers will no longer sync past 70 taken

I have cleared customer data and have tried to re-import, they say they are taken for the early order customers up to 70 and the process just stops when it checks the first 70 users using default settings. I am trying to sync the last orders in the last month but it will not go past 70. I do not see any errors in the log etc. Any thoughts on this? The first 70 say that they are taken but the process just stops.