Run your .iso through DMToolbox (came with DM 2.8) to generate a set of output files.
Place the genreated directory (will be the 6-byte game ID code) and all files inside of it into the USB:/games/ directory.
See if that helps.
What version of WiiFlow are you using? Only the latest (4.1) will work. You can also try to use GX r1209 and CFG r51 (links to all three of the loaders that I mentioned are in my signature).