Which version of RCT do you have? Some workbenches are made w/ Loopy Landscapes, and if you have CF/AA, they won't work because of the new attractions. If that isn't it, try un-installing and re-instaling the game and patch. You won't lose any saved games if you do that. Hope that helps!
~TodD~
"God help us, our lives are in the lives of engineers..."