That’s been happening to me too. So I’m learning to check in with my box elbow and see if it is too high for the string I am on. I also heard one teacher say to aim for the back of the left foot. I think that was meaning that perfectly straight bow might mean the arm is different ( has a different aim/angle) on the lower strings vs. the higher strings.
And finally I have checked to make sure I have proper rosin on the bow and that my strings are wiped fairly clean. You can use wipes for eye glasses every once and awhile, but cover the fiddle so nothing drips on the finish. Otherwise a dry cloth after every session on the fiddle is a good way to keep strings and fiddle clean.
