Passing parameters can be very complicated to make sure it happens right. It really isn't possible to debug it over the forum. Usually, when you are getting prompted for parameters, then the way you are passing them to the report isn't matching up with what it is expecting. It is easy to get the parameter name wrong (I even think it is case sensitive). Plus, you have to make sure you either just use the viewer object model or the report object model. If you try to pass info to the report using both models then the report object model will overwrite the viewer object model. See, lots of stuff to think about and it can get pretty complicated!  I recommend not using parameters and using formulas instead. Thosse are are easy to get working and never fail.

Hopefully, this might help clear things up a bit. I have a ton of sample code and info about parameters in my CR.NET book. But the best way to go is with formulas.