I faced this problem where I used a new flash trigger and after a couple of good photos, photos started appearing like this https://imgur.com/a/iNw4rkr then the camera stopped working for a while and when it did there were two lines following the f stops. so I tried the old trigger and had the same problem, after a while I realized that the shutter was 1/ 400 and 1/250, and when I slowed it to 1/100, no problem anymore.
You need to make sure your flash unit can keep up with your camera shutter speed.
I’m not sure about the specifics, but I think HSS (High Speed Sync) will allow you to avoid those situations. But your flash needs to support it.
If not, just manually change your shutter speed to what the flash permits.
The flash can only operate if the entire shutter is open for a moment. At shutter speeds above about 1/250 second, the second shutter curtain drops before the first is all the way at the bottom.
Your camera will list a shutter synch speed. That’s the fastest you can shoot and fire a flash.
Make sure HSS is enabled everywhere — on my setup I have to enable HSS in the camera, on the trigger, and on the flash unit.
Models of your gear would help. We aren’t looking over your shoulder.
But based on the information you gave it’s likely a flash sync speed issue.
The time a flash is emitting light can be very short, shorter than 1/100s. For example, the Godox V1 reports a flash duration of 1/300-1/20,000s.
If you look at high speed video of camera shutters, you can see that the camera has two curtains, the first one opens, exposing the sensor to light and then the second one follows it, blocking light again. At slower shutter speeds, there are periods of time where the entire sensor is visible. However, once you get to a certain fast shutter speed, only part of the sensor will be exposed at any one time with both curtains moving at the same time. Because the flash duration is so short, if it goes off during these faster shutter speeds, only the small slit that is exposed will see the flash, resulting in the black areas you see in your images. You can see that as the shutter speed gets faster, the amount of black you see should increase.
So the fastest shutter speed where the entire sensor is visible is your flash x-sync speed which you can find in the camera specs/manual. The alternative is to use a High Sync Speed (HSS) flash mode, where the flash will go off multiple times to extend the duration the flash is emitting light.