Before downloading the map from Hive Workshop, be sure to read the "Recommended Version" tag in the description.
If the version is higher than the current one (1.31 or even 1.32+ aka "Reforged"), then the map is, you guessed it, unplayable on the older versions.
Also, another possible reason for blank custom map is that your download is corrupted. If this thing happens, you should re-download the map again, making sure they are placed in the correct directory. For 1.28+, they must be stored in "Documents/Warcraft III".