Hey Karin...
I'm thinking Josh's answer is probably the best start... and if not, perhaps just checking to see if there are any other rules applied to that field in SF, that would prohibit the field from being updated?
Just a question.... and I'm guessing this ISN'T the culprit, but worth a shot, just in case!?
When you collect the data in Marketo, is the field updating and saving with the new value? Meaning... you get the data, and see it register in the Activity Log, but is the "Change Data Value" to that new value being skipped? Is the new value populating/saving in the lead's record for that field? If the value doesn't seem to be updating in Marketo, it may be because that field is being blocked form updates from forms? If that happens to be the case, then the field isn't truly being updated, and SF wouldn't see a change.
Good luck!