I'm using dedicated DX communication commands to communicate with the Mitsubishi MELSEC. The first command is accepted, but I get no response from any subsequent command (even the same one as the first command). What could be the problem?

Some commands are sent in units of words. This always results in an even number of bytes. But if you send the command "FD0,1,1CRLF", for example, that is an odd number of bytes (9), and the 10th byte which follows CRLF will be populated with "junk." This junk byte becomes the 1st byte of the subsequent command, making it unintelligible. Try inserting a space as in "FD0,1,1spCRLF," to make it an even number of bytes.