The most common reason for an application to consistently fail to download (the particular VI may vary) is that the program is too large to fit in the memory space of the [c]FP-20XX module. If your program is borderline too large, you may try re-booting the [c]FP-20XX and downloading again (this will free up memory that may still be allocated from downloading and testing the program previously). If the program is still too large to download, you will need to either a) optimize your code to be more memory efficient or b) remove some of the code from the downloaded program and put it up on a client machine.
Regards,
Aaron