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Hi Grace,
Sorry I misread your first message. If you’re getting black lines at 1/250th you’re definitely not achieving your camera’s expected max sync speed, which I would indeed attribute to the flash trigger.
Which one are you using?
Some ‘smart’ triggers (TTL / HSS capable) have various modes and you have to pickup the right one for your strobes.
If the trigger isn’t working up to specs, there might be a chance to get its firmware updated (not sure).