DIAGNOSIS: You're using the adjustable panel incorrectly. It's designed to extend horizontally to fit the window opening's width, not as a vertical filler.
FIX: The panel extends sideways. Loosen the screws on the panel's sliding mechanism. Pull the panel outward horizontally to match your window's width. Tighten screws. It fills the width, while the included foam panels fill the height gaps above and below.
IF THAT DIDN'T WORK: For vertically tall openings, you use the foam panels that come in the kit. Cut them to size with scissors to fill the space above and/or below the plastic window bracket.
HEADS UP: The goal is an airtight seal, not a perfect mechanical fit. Use all the foam and sealing strips provided. Any gap, even half an inch, lets in hot air and cripples performance. Tape can be a temporary sealant.