I solved a similar problem by passing the value of the available parameter in the URL instead of the label of the parameter.
For instance, I have a report with a parameter named viewName and the predefined Available Values for the parameter are: (labels/values) orders/sub_orders, orderDetail/sub_orderDetail, product/sub_product.
To call this report with a URL to render automatically for parameter=product, you must specify the value not the label.
This would be wrong:
http://server/reportserver?/Data+Dictionary/DetailedInfo&viewName=product&rs:Command=Render
This is correct: http://server/reportserver?/Data+Dictionary/DetailedInfo&viewName=sub_product&rs:Command=Render