You need to check out the correct settings as the flash sync speed for you 6D ii is 1/180th which you should be able to use at 1/200th so only being able to get less than 1/160th would indicate incorrect setting in the external flash mode or a slight fault.
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You need to check out the correct settings as the flash sync speed for you 6D ii is 1/180th which you should be able to use at 1/200th so only being able to get less than 1/160th would indicate incorrect setting in the external flash mode or a slight fault.