I updated the supplier lead time and it hasn't updated some of the items - why?

The lead time is applied based on the priority in this article: https://stocktrim.zendesk.com/hc/en-us/articles/360057662734-Where-does-the-lead-time-come-from-

So if you have ever saved the lead time on this item explicitly then that will take precedence and the supplier default will not apply. To update these items you can either simply save them explicitly, or use the bulk csv update method.