You can try putting some archival foam core behind the mount board to stiffen the back a little bit. But truly, if you want fiber prints to lay perfectly flat in your window mat, you really should think about dry mounting.
If that's not something you want to do (I don't like dry mounting, and framers at a couple of big museums have told me not to), then consider using a stiffer backing board that will provide more force against the back of the print.
If you go to museums and look at vintage prints, many of the ones I see don't lie perfectly flat either. My own prints are framed and matted using hinged window mat, and a print mounted using nothing but photo corners. With the seasons humidity changes, the different materials expand and contract, and in the low humidity winters we have in Minnesota, the gap at the window mat is always a bit bigger. Not a big deal, honestly.