Another thing to remember is that some DLLs (for example RxFtp.DLL, RxSock.DLL) only export a general routine to register all other functions from the DLL (e.g. FtpLoadFuncs, SockLoadFuncs). In this case you can not register functions from the DLL manually.
You may use a tool like ExeHdr to determine what functions are exported by a DLL.