Symptom:

When assigning work to individual students work, they do not appear on the class list.

However, when class wide work is assigned, they receive it the task.

This can sometimes happen when you your browser cache needs to be cleared.

Data sometimes just conflicts with each other, and it just doesn't show. 

You can try a logging onto a different browser (if another one is installed on your computer) or try on a different device. 

If this device shows that the students are ALL on the work assignment list, you may need to just clear your browser cache.

Solution:

You can find the instructions on how to do that here.