SSRS Auto refresh stops working
The documentation states that you should not enable snapshot isolation on SSRS database.  

Check here: 


Enabling Snapshot Isolation on the Report Server Database.
You should follow the instructions there to turn it off.  

Excerpt from the docs:
"...You cannot enable snapshot isolation on the report server database. If snapshot isolation is turned on, you will encounter the following error: "The selected report is not ready for viewing. The report is still being rendered or a report snapshot is not available."  


As for the cause of the blocking, and ways to fix them, here are some links to help you:  


SSRS Blocking Processes
SSRS issues locks to provide a multi-threading mechanism. Once all threads running the report that you wrote, end, the lock is removed. So if you see these blocks regularly, it is usually due to a slow query.
Advice on setting up a Reporting server
(advice on how to set up a second reporting instance)