My suggestion is to either debug your code or modify it such that it does logging. For example, add a try/catch around the call to the unmanaged code and log any errors.
The problem might be that the logic is not finding your DLL. Did you put in in the same directory as the executable image which is running, e.g. ArcMap.exe? You should be able to modify the System PATH environment variable and put it in whatever directory that you choose. Just note that existing process will not see the updated environment variable unless they are restarted, e.g. services.