Learner assignments not getting created

Summary:

When trying to create learning assignments for employees in bulk whether adding them manually or using bulk upload of csv file we are noticing that some of the employees are dropping out and are not getting the assignment.

Content (required):

So for example if we are trying to assign 20 people to a course , assignment is getting created for 10-12 people and for the rest the assignment is not getting created. The only co-relation we have been able to find between people for whom the assignment is not getting created is that they do not have a user account. Although they are active employees. Has anyone faced this issue earlier.