after adding Crystal reprots class library isnt reffred.

i have a windows application which have multiple class liubrary

windows application:

1.           User class library
2.           Inventory Class library
3. purchase Class Library
4. Sales Class Library
5. Accounting Class Library
6. and more other according to their Functionality

i try to use crystal reports in sales class library to make print layout for the sales bill. after adding and creating reports when i finally complied i find the application where i created instance of Sales class library the reference is invalid whether the .dll for the same class library already in reference. if i delete that reports generated there in that sales class again the .dll is reffed perfectly in main project.

i didn't understand why it is and didn't find any solution to solve that. plz help me regarding this.