Hi there,
Do you have multiple Header elements added?
If so, the 2 Header Elements may be conflicting with each other.
Another potential reason is perhaps the Header Element is working but you’re not seeing it within the page because it has no text content. Meaning it has nothing to base its height on.
To check, add random text and/or padding top and bottom value to the Header Element so it get a height value. You then check them on the live page to see if it is visible.