There are several reasons why you might experience these problems with Handouts:
1) If your classroom internet connection is weak or has a problem
2) If you or your students' devices are not running the latest version of Handouts and iOS/Android.
3) If your school has not whitelisted the following domains:
Any request to s3.amazonaws.com or any subdomain
Any request to scalabl.cloud or any subdomain
4) If you and your students are not using the same platform, i.e. all using Android or all using iOS. 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 you are registered on (iOS/Android), the PIN code for the class in question and the assignment that is not showing. We will look into the problem as soon as possible.