If you are using a generic thermal (ie. cheap) flasher unit, most likely the lamp is not drawing enough current to cause it to flash. (2 lamp thermal flashers generally require at least 4 - 5 amps to flash, equivalent to a pair of 1156 bulbs)
Solution is to can wire another lamp in parallel to put more load on the flasher. Or swap the flasher for an "electronic" flasher that's not current sensitive.