There are two possible issues that could cause this:
1) you're developing on the VS.NET webdev server or IIS6, but deploying on IIS7. If you're deploying on IIS 7, you need to add the IIS7 config elements referenced on this page: http://krystalware.com/Products/SlickUpload/Documentation/concepts/iis7.aspx.
2) you're running SlickUpload on multiple servers without using a status manager. SlickUpload supports a clustered configuration, but requires configuring a StatusManager to store upload progress in a central status store so it is accessible from all servers. Take a look at the Clustered sample for an example of using the built in SQL Server status manager to store status in a SQL Server database.
If neither of these seem to be the issue, email your view code, controller code, and web.config to chrish at krystalware dot com.