There are several reasons why your students might not be able to view the assignments you sent out:
1) If your classroom internet connection is weak or has a problem.
2) If your students' devices are not running the latest version of Handouts and iOS/Android.
3) If your school has not whitelisted the following domains:
4) If you registered as a teacher on iOS your students will not be able to access assignments on an Android tablet, and vice versa. The iOS and Android systems are not compatible at the moment, but we are working on this as we speak and hope to have the functionality ready for early 2015.
If none of these seems to be the problem please get in touch at email@example.com, letting us know the account details for you and the student(s) in question, what system they are registered on (iOS/Android), the PIN code for the class in question and the assignment that is not showing.