After much frustration, many times over the years, I spoke with a very knowledgable rep who said the two most common causes of points transfer delay are:

-100k+ transfers: some get flagged for review
-name discrepancies: any small differences will cause problems. Chase can see your name and the mileageplus name, so if you call and ask they should be able to tell you if there is a match or not

There is a team of people reviewing these cases manually once flagged. Chase has limited ability after 24 hours to do anything other than make a note on the account if the Chase rep happens to see the issue.