Values for Custom Fields not Stamping over Previous Data

[AshleighMo]

Ok what am I missing? I have a very simple program set up to stamp UTM values for ungated web visits. The program is working as inspected, triggered by the action and stamping the value to the activity record. However, that's where the success ends. The lead utm_source field is not updating over the previous data in the field. 

1. Can you not overwrite the information already in the field? 
2. If you can overwrite it, would an API push of data trump a data change in marketo? 

For this scenario May 1st was the initial utm_source stamp "Speaker" and then today May 9th the lead was stamped "Email". The field is essentially supposed to be the last touch so we want it to change as they interact with other channels from us.