In moving to Constant Contact from a previous email marketing platform, I was trying to Add Contacts From File using a 2MB .csv with 6,225 contacts. Only 1,670 were actually being added with 1 error (pretty far off from 6K).
Re-saving my contact list as a .xlsx file type and re-uploading provided a workaround for the issue.
I wanted to post this up here to hopefully help other Constant Contact users experiencing the same problem and to bring the issue to the attention of the Constant Contact Support Team.
Thanks for posting your workaround. Sorry to hear you ran into that problem! When it comes to uploading your file, either file type should be uploading the same amount of contacts. If you would like to email us your file, we could take a look at it and see if there is anything in the .CSV that would cause the system to think there are only 1,670 and not the full 6k.
Just reference this post and you can send your file to social_support(at)constantcontact(dot)com.
We would be happy to help!