Is the parameter defined in your dataset/shared dataset?

Sorry, misread your post. I had this issue once and the problem was with the shared dataset.....

Have you made sure that the names of the parameters are exactly the same? If you have changed things as you went along then you might have missed that out?

Have you tried removing the parameter from the drillthrough report and then recreating it? Sometimes when copying/re-purposing reports things can get corrupt/mixed up...