flash can be brighter in 2 ways.. 1) increase to a faster shutter. 2) reduce the distance to the subject. I assume you have already done 3. which is flash is at full power.
at ISO200 setting the flash at 1/1, F22, 1/180, 2-5m to the subject. the flash would be brighter than the sun even under direct sunlight. if you have D70/D1 series. 1/500 would make the sun even darker.
F8/2000.. sun isnt that bright..
brightest sun can be.. ISO100 F22/125.. (no clouds)
ISO100 F16/125 ( with clouds)