It could be a few different things.
You may want to look at the SSRS service logs to see if you have any errors. The files are located at:
\Program Files\Microsoft SQL Server\<Report Server folder>\Reporting Services\LogFiles
The files are named ReportServerService_<date_time>.log
Most of the time errors are located in those files. And what to do depends on what the error is.
You can also try using Internet Explorer, right click on it and select Run As Administrator. Run as administrator will respond differently than just being in the Administrators group. Those are two different things.