Issue with Importing Players

Whenever I create a new sandbox league, I'm trying to import my own custom players. However, each time I do it, approximately 1,000 of the 3,500 players are not imported. I've gone into the csv files to check for any errors, but both players that are imported and those with whom I am having issues imported show no differences.
I've tried troubleshooting myself for the past week with little success. I did find out that importing again after the creation of the league will import the missing players successfully. However, this causes another problem; after saving the league, when I try to load it up, it gives and indexing error.
Do you have any ideas what could be causing this issue?
I've tried troubleshooting myself for the past week with little success. I did find out that importing again after the creation of the league will import the missing players successfully. However, this causes another problem; after saving the league, when I try to load it up, it gives and indexing error.
Do you have any ideas what could be causing this issue?