You may be encountering a recent issue that has developed in EC2.
(From the Pre Built AMI's launch page...)
An update to the EC2 Management Console prevents the Wowza Media Server AMIs from launching properly. The workaround is to specify the default startup package using the User data field as part of the AMI launch process. To do this:
In the Configure Instance Details step, open the Advanced Details section.
In the User data field, enter the following user data and then select the As text option.
You may replace your startup package (if applicable) using the same approach from your download source.