Since it is unlikely that the leading curtain will have magically sped up, it is likely that you need to increase the tension in the closing curtain. Try one or two "notches" - that will make sense when you look into the work.
More than 1 or 2 notches, and I would say that the underlying issue needs to be investigated.
You can film the shutter to check accuracy using an iPhone on slow-mo. Not particularly accurate, but should be good enough to check evenness.