Would you provide more info on how this data is passed to Marketo?
Remember, DVC may not trigger on net new leads.
+1 on new records being created not causing a Data Value Change.
Do you have screenshots of the trigger and the activity on a log? Are there any other filters that are preventing these records from flowing through? I think campaign code is a custom field so any caveats might be limited to your specific instance.. But we definitely need more info to diagnose.
You could also check whether the DVC is actually a new value.
E..g, if someone fills out the form and the campaign code is the same, the DVC will not actually register as a trigger for the campaign. (Although in that case you wouldn't expect to see it in the activity log as a DVC either).
Regardless I would agree to check for filters messing with your logic as Dory said or if the lead is first being created you need to do Person Created Trigger + Campaign Code filter to make it work as Josh said.
Any filters on that campaign? Qualification rules set to "every time"?
If you create a lead and manually change the Campaign Code in the lead detail page will it work?
Also, may sound silly, but check if there is a confusingly similarly-named field that may be actually receiving the DVC. Sometimes that has tripped me up before.