The likely cause of this is because a particular index field for a document is blank and that index field has “Required to Continue Importing” turned on. Required to Continue Import will load documents in a linear format. Once it comes to a document that is required to have a value, but is left blank, it will stop the load process and will not finish loading the rest of the documents until that document is properly indexed.

The Required to Continue Import feature can be found by opening the Admin Console, going to the properties of the project, selecting the particular index field and selecting Edit. In the bottom left corner , there is a check box for Required to Continue Import.