In the image below, all the dimensions are of the same style and settings. But the ticks have disappeared from the dimensions on the left. The ticks are custom, by the way (rhino’s are too thin), if that makes a difference.
This problem appears often but seems to always correct with project close/open.
update: The styles aren’t the same, but there’s definitely an issue. The correctly-displaying one is a duplicate of the tickless one with only the tolerance changed. Somehow the user arrow definition in Options for the misbehaving Dim style had become unassigned. Rhino perhaps did this when I recently duplicated the style. I reset it, but this problem has arisen and corrected itself in the past, so something’s acting up.