You have a section break not set to link to previous, I suspect. However, you cannot simply reset this or your earlier (previous) headers will disappear.

Go to the earlier section and double-click in your header on an even-numbered page. Select and copy the entire header.

Go into the chapter that is the problem and double-click in the header on an even-numbered page. Select the entire header and paste the header from the earlier chapter.

If the correct Chapter name shows up you are fine. (It should if a StyleRef field was used.)

Sections / Headers and Footers in Microsoft Word 2007-2013