This may happens due to the following:
Possible causes:1. I think that you have to delete the files you want to replace just before you copy them. ASP.Net tries to pre-compile everything it is trying to serve. There is an issue if you just copy and replace, but delete and then copy seems to work.
2. The dlls are compiled in different TimeStamps(See this[^]).
3. Unlicensed dll(Get a licence for your ddl[^]).