Manual Mode of the camera or Manual Flash mode? Different things. What do you mean with "i use 1/8"? 1/8s shutter speed or 1/8 flash power?
If you use flash power 1/8 and (based on all other settings like Aperture and ISO) the resulting flash speed is above 1/200s then the camera will set the speed to 1/200 and the shutter speed display in viewfinder will blink (Canon). Reason is that 1/200s is the maximum X-sync speed for flashes in normal mode. You can take images but the result might not be what you intended.