Usually, this is due to using the nose of the airplane as a reference.

When I have a student doing this consistently, I draw a little cross on the
windshield with a whiteboard marker directly in front of his face. Then I
tell him to use the cross as a reference instead of the nose. Works every
time, and the whiteboard mark wipes right off.

I also use whiteboard marks on the windshield for other purposes, too, such
as getting Microsoft Flight Simulator junkies to look out the window, and
when students are having trouble judging altitude on final approach.

ZZ, CFI.