Will you look in your campaign queue to make sure there are no leads remaining in that campaign? You can find it by going to Marketing Activities and then clicking on the "Marketing Activities" folder in the left-hand pane.
Another question: are you sure that all 15k of these leads did not already have that value? Is it possible that the rest of the leads had the desired data value already? Use the search bar in the Results tab to search for "skipped."
Show us screenshots of the smart list and results tabs. What does the Last Run count say?
It is possible a bug caused it to break or the If Statement isn't quite what you thought.
Thank you Rachel & Josh!
I figured out. There was a bug with our vendor that the field was related to. When I first ran field updates, the new values were stored in Marketo, but were not reflected in SFDC. So, when I tried the second time, MKTO recognized those values have already been in there. However, I don't understand how come MKTO didn't show "Skip" in the results.
Anyways, I changed the value to a totally different value, pushed to SFDC, and wait for few minutes and changed it back to the value I want it to be. It all works now.